one advise i want to give, from my small little experience in this
first, find out the limits of your 4g15 engine, research and know the weakest points that needs to be taken care off
then secondly, know yourself what kind of power you are expecting , and how hungry you are
please do know, there are few 4g15's out there are really powerfull , and bare in mind its not the supercharged or turbocharged ones , in the past there were a few too in the batu 3 tracks, and they were built for races not by thousands but 10 and 10 of thousands :D , just saying, to build a really all out machine on a 4g15 would not be cheap because of custom fabrications and lack of on shelf products
but if the power that you are looking for with a good budget , then i would say go for it. Just afraid that you will hunger for more power soon :D