If your current ESC can handle 8.4v, then you should be OK with a Lipo (although probably avoid the higher C ratings). However, I tried the same on my return with an LRP IPC v6 and Trinity D3 11x2 and it worked OK but I scorched the brushes in just a couple of runs. After that, I took the plunge into brushless and the difference is night and day! Much faster, more power and virtually zero maintenance - was a win:win for me!
You won't really go wrong with any of the main manufacturers (Speed Passion, LRP, Novak, Tekin, Orion, etc). I would recommend matching ESC and Motor brands for best results, although this is not strictly necessary. Your Bosscat transmission will probably struggle with anything much hotter than 10.5, so bear that in mind too. There is a new Cat on the horizon (SX3) so you may prefer to wait for this (although none of your existing wheels etc will fit). Alternatively, look through this forum for advice on which other 4wd's might suit your needs.
Depending on your budget, you will be able to source good examples of all of the above in the "For Sale" section on this forum too.
HTH
