Hi Ksionc - Welcome to the forum and the mad world of RC tanks - actually it's a great hobby and very fulfilling but can be frustrating, maybe expensive and time consuming but it's a hobby so mostly up to you. Just enjoy it and don't take it too seriously.
Now, with regard to your choice of starter tank. the M26 or snow leopard isn't a bad choice. I have one that I have modified a bit from stock and it runs okay on metal tracks. Like a lot of the HL models - they are not totally accurate and will need some modifications done to get them more correct. The M26 has a too short gun barrel which is one of the mods I corrected. their are a few issues which I've forgotten about now but to me they were not of interest so I left them as is. the RX18 is the standard HL MFU which replaced the RX13 & RX14 boards. The RX18 is a good MFU in my opinion and can do a lot but their are now other control boards and hobby grade radios on the market which you can upgrade to if desired.
Upgrading to metal parts is optional and you will get a lot of discussion on here about it. I have had a go at most of the upgrades and for me, it isn't necessary on all the models or desirable. The metal adds weight and puts strain on the drive train and will drain your battery quicker. You can fit better, lower ratio gearboxes that will give you more torque and slow your speed down but this may not be what you want on a modern tank. they are also noisy and can drown out the speakers volume so you need to enhance the sound with a better speaker and amplifier etc.
So, I would go with stock (no metal) HL M26, Panther G, Sherman, Leopard or M1 abrams as the quality on these tanks is much better now from HL . You can then upgrade as you desire and search the forum - ask for advice as you go.
Good luck and happy tanking
CaptB