as said avoid BAM and AMK lumps as they cost far too much money. i would actually suggest getting yourself an AGU (150bhp) engine, as these are the cheapest of the lot. if you buy a pre 99 engine, it will also come with forged rods, and 20mm wrist pins
then add a KO4 manifold and turbo setup, bigger injectors (saab redtops are about £60 a set and will run 400cc each at 4bar),BAM/AMK inlet manifold,forge 007p Diverter valve, FMIC and a remap, will put you well over 225bhp for less money than you can buy a BAM engine for!
Cheapest engines around seem to be the AEB lumps from Passats, but you will need to change the Sump/oil pickup and also the inlet manifold to fit into your polo. But they are large port non VVT engines, which is what you want if your going to be tuning it.
Saying that, i have seen a few AMK engines on ebay recently for not massive amounts of money, and lets face it, 210bhp in a polo wouldnt be shy!