15k alone on engine? ... That's gonna be really sweet..
But if 15k alone upgrading the car from stock standard condition, then it's not veeeery much.. more than one third of your funds will already go to your engine swap, then you still have to consider other upgrades like brakes, rims and tyres, suspension setup etc.. And you'll spend a sum on accessories as well along the way.. If you're inexperienced, you probably will also spend some unnecessary money and make some wrong buys.. Yada, yada.. But you'll still be able to get good upgrades with that budget and build a good car.. But that's for another thread, the upgrading part.
On your question, ecu.. Based on my limited knowledge, stock ecu is sufficient for street performance. If you have extra budget, go for piggybacks like E-manage. Even deeper pockets, go for standalone ecu like Freedom, Microtech, Haltech etc, just to name a few. 15k to spend on ecu? Maybe can get Motec.. But "street performance", I'd think stock ecu is more than good enough to start with.
Gearbox.. 6 speed is nicer to drive than 5 speed, and since you have 15k budget I think 6 speed should not be out of reach for you..
My humble 2 cents.. Other sifus please correct me if wrong..
