It's a single scroll stock location turbo. You do NOT need new manifolds, up pipe, down pipe, etc. The only difference fitment wise will be the intercooler, you must get one that will be compatible with a sti outlet (PW tmic, any fmic). Depending on whether the compressor inlet is 2.4" or 3" you may need the 3" inlet pipe which will necessitate TGV deletes, manifold removal, etc. Looks like it comes 2.4" but you can request 3" inlet.
Sleeved block? For only 480-500 bhp?? If it's rated at 480-500bhp I'm assuming it flows around 48-50lb/min, certainly not sleeved block territory, but definitely enough to break your ej255 bottom end.
Looks like a solid turbo and a reputable company, but I have no experience with them, and have never heard of anyone who has. At the end of the day though, you'd be foolish to spend that amount of money on this turbo, when you'd spend less, and likely get a better product by buying from someone like Blouch. If you wanted the most "true to Garrett" stock location turbo you could get, go with ATP.
If you haven't done your research, YOU SHOULD. This turbo is certainly larger than a 20g, and will obviously require a laundry list of supporting mods.