This thread seems to have moved from gearbox recommendations to full modification recommendations.

I know what you are asking for - a good "turn key solution", but unfortunately, there isn't one, other than dropping $500+ on a high-end ready-to-run tank. And in the end, you'll end up spending more than that trying to figure it out yourself.

I've been dabbling with RC tanks for a couple of years now. I buy only Heng Long tanks. I've got 2 Jagdpanthers, a Sherman, and a PIII. The PIII is a fixed-crystal, ABC channel radio with no smoke and no sound. The Jagdpanthers are RX18 units (I think) with removable crystals. They have (had) smoke and sound. One tank has plastic tracks, another has metal drive/idlers and metal tracks and Imex gears. The Sherman is all plastic with 2.4 GHz radio.

I'm in the middle of rebuilding the Sherman. I got an IBU2 Base board, and TARR smoker. I'm doing a full custom paint job/rebuild.

If you are looking for a "turnkey" package suggestion of how to custom build a tank, there isn't one. It's all very confusing to me. I delayed buying anything for a long time because I didn't know what to get (it's called "decision paralysis" - it happens when there are too many choices). Finally I just bit the bullet and bought some stuff so I could figure it out as I went.

There are numerous control board choices. You've got Elmod, Clark, IBU, and Open Panzer, just to name a few. There may be others. Then some of the brands, like IBU, have different flavors, like IBU, IBU2, IBU2 Base, IBU2 Pro. Clark has TK22, TK24, etc. etc. etc.

What's the difference between them all? Which ones are "best"? I have no freaking clue and I've been reading along here for a couple of years.

Some provide better support/compatibility with different kinds of IR weapons systems. I don't do IR battles so I never worried about that. Some have built in control circuitry for motors, others require separate controllers for motors (ESC boards). Some have built in sound capability, others do not. Some have programmable sounds, others do not.

If you want to get into building your own tank internals from the ground up, basically, you will need to research and consider the following things. Take all this with a huge grain of salt because I feel like I have only the vaguest of understanding of these things.

Transmitter
It's going to be 2.4 GHz. Some are programmable "stand alone", others you have to hook up to a PC to program. Others may not be programmable at all. The more channels the more money and the more options you have. I bought a HobbyKing "generic" 6 channel radio for my Sherman project. You need to buy a separate cable to program it with a PC. Also, not all transmitters have a "return to center" stick. You'll need to research and understand the difference between Mode 1 and Mode 2 transmitters.

Receiver
Your transmitter needs a matching receiver, and the receiver needs to work with whatever control board you are going to use.

Control Board
This is the "brains" of the tank. It usually provides sounds out to a speaker, but not all control boards do. They are sometimes programmable to provide different driving performance characteristics, like slow start up and slow down, to simulate the inertia of the tank. Sometimes control boards directly wire to your motors, as they have ESC (Electronic Speed Control) circuitry built in. Other boards require separate ESC boards.

Speaker
Lots of choices and people go to a lot of trouble making custom speaker boxes to try and get the coolest sound.

Smoker
The TARR smoker seems to be one of the best out there. The Heng Long smoker won't work for very long, and I've tried some other aftermarket ones and they seemed to be just re-worked HL smokers and I was not impressed with them either. With the right control board you can get "proportional smoke" that produces more smoke with increased sound/engine speed. Makes it look like a real engine is operating in your tank.

Gun
Heng Long only makes BB tanks nowadays. If you want to do IR battling you will need to figure out who's system you want to be compatible with (Tamiya, etc.) and figure out how to wire it all up. I haven't paid much attention to this since I'm a solitary tanker. Some people go to great lengths to make custom gun arrangements that provide realistic barrel recoil.

Gearboxes
Many HL tanks come stock with plastic gearboxes. These often have built-in slip clutches that prevent over-torque and gear stripping. These work pretty well in lighter tanks like the Sherman, but the clutch will slip a lot with heavier tanks like the Jagdpanther. If you want to run metal tracks, you will want a metal transmission. Beware the Heng Long zinc (pot metal) gearboxes. They will wear out in weeks. Various folks make steel geared boxes and they work well. The whole "gear ratio" thing is a giant can of worms. Somehow people started calling these 3:1, 4:1, etc. etc. when this has no bearing on the actual gear ratio of the transmission. Personally I hope I'm able to sort speed issues by programming the IBU2 rather than worrying about the gear box "ratio". If you want to go real custom you can get one of those ETO Armor transmissions with the billet frames. They cost as much as some Heng Long tanks but they are said to be bulletproof.

Tracks
Metal looks cooler than plastic. With metal tracks you'll probably want metal drive and idler wheels.

Track Tensioner
These are nice as they let you adjust the right visual amount of "sag" in your tracks without playing with adding/removing links. With some tanks that are prone to throwing tracks they could be considered essential.

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Each one of the above things often has multiple options to choose from, and the choice you make for one thing can dictate your choice of options for another thing. And everyone does theirs a little different. So trying to get someone to recommend a "package" that you can just go buy and use is very hard. And really if you want a "turn key" solution you're probably better off dropping $500-$900 on a RTR high-end tank like an Imex or something where all of the integration of the different systems has been done for you. Honestly you will probably end up spending at least as much money trying to do it yourself.

Here's a great example:
Taigen Tiger 1 Late Verison (Metal Edition) Infrared 2.4GHz RTR RC Tank 1/16th Scale

$690 for a nearly all-metal tank with metal tracks, all metal wheels, metal suspension, has recoil, smoke, sound, 2.4Ghz radio, and more.

Now, you can buy a Heng Long Tiger 1 for $200. But if you start pricing out all the above upgrade options, you will discover that you will very quickly start approaching the $700 of just buying the ready-to-run. Not to mention that you then have to figure out what pieces to buy to work with what and hope you buy the right stuff the first time. I buy mine in pieces mostly to keep the cost of the tank under the radar with my wife. I'm like Johnny Cash building his Cadillac "one piece at a time".

Steve