I would go brushless first. You will get longer run times on your
Nimh for praticing. You also get the lipo cut out built in to the esc.
The best reason to me is that you can buy good packs of Nimh
from racers that are converting to lipo for a fraction of money
for what they payed for them. Then wait till you have the funds
or more lipo competition is out in the market driving prices down a bit.
On a side note a 13.5 is quite quicker than a 27t. We run 17.5 to 27t
for stock here. Then jump to modified which is a 10.5 or 19t.