Ok so it is my turn to chime in here with a few points.... first of all, these RCU guys know what they are talking about. I work in the RC tank business and I make it a point to research other tanks and mods once I hear about them. Let me tell you, HE is one of the most difficult ones to upgrade from what I gather. I personally myself do not have a HE tank to compare with, but I trust these guys and have seen some of the difficult build logs for these tanks.
With that being said, a set of gearboxes, tracks, and drive/idler wheels you have already gone past the total value of this tank. So be aware of the cost beforehand, I have done that too many times Also, a RX18 can be run off a 2 cell lipo. This is a point I try not to get into, nor promote. NIMH is perfectly fine for most all activities in a RC tank. Also most motherboards will not have a LVC for lipo, but for NIMH. So even if you can run a lipo, there is a chance of completely ruining it on the first run as you have no LVA or LVC to warn you. Now, a LVA (low voltage alarm) is very cheap and is put on the balance lead of the lipo. When it detects any cell reach a predetermined value you set (usually 3.0-3.6V) it goes crazy and starts beeping to warn you voltage is low. Once again, lipo is not needed, but will work for most boards. Please consult with your manufacturers on their recommendations first.
Also, if you want to see the HL Abrams, here are a few pics. At the point when I got this, there were only 2 of them floating around.

As you can see there is plenty of room to work with on the inside. Also Taigen will be making upgrade parts Can anyone say metal lower hull?