lol that question on this forum, considering the such diversity of routes to go in tuning is like asking how long is a piece of string
on completely stock internals, a tailored chip, like Jabbasport £450 or Revo which starts at £500 i believe, will get you to 190-200bhp
if as you said you are on a budget then that is the "most" you will get
you dont need to change over any stock parts
my personal choice and in my opinion, I would go to Jabba, as when i had the sp1 revo chip on mine, there was too much torque and alot of heat soak, therefore the power died off far too quickly
jabba i would recommend to
help spread the power over the rev range a little, but it comes down to the physical turbo i.e. it can only pass so much air and you will only ever get so much power from it
IF you are on a budget, to me i would presume £1000, as thats prob the cheapest you can go for a reasonable selection of modifications i would get the following (as you already have the DV and modified airbox):
Downpipe & 100 cell race cat
S/S exhaust
Remap
the downpipe is the biggest strangle point on the ibizas, one of the best upgrades to get really if you want to unleash a nice bit of power on the standard KO3 turbo and
help it carry on further up the rev range
although what i have just quoted is a little over £1000, but if you get in on one of the group buys for the race cat and exhaust that would sure
help you get things on more of a budget