i have an ATP turbo as does @
MPS2NV, @
rayd did (now EWG) and @
LittleredMPS is joining the ranks soon
it depends a lot what your goals are... the ATP hotside, ie the bit that bolts onto your manifold + downpipe is fine as long as you arent planning on going too crazy (ie going past stock fuel system/300kw at the wheels)
if you go non-stock flange, ie T3/T4 you'll need a new manifold and downpipe, and may as well go external waste gate whilst you're at it... but it opens you up to so many more options for turbos... unfortunately we dont have the ease of custom making parts like they do in the states, so it wont be cheap...
as for TIP/MAF, i would suggest getting a bigger MAF housing at least (where the sensor plugs in on your intake) else you cant really make use of the turbo effectively... as it will draw in more air than the sensor can read and you'll either have to de-tune car, or it'll have MAF cut and scare the bejebas out of you
the easiest way (IMO) to upgrade your turbo (at least for 90% of people here) would be a BNR stage 3... unless you are planning on pushing more than 300kw at the wheels, there is little to no point in going bigger