wastegate sizes please!

Wastegate can't be too big, if you find a 44mm you like, go for it. You could run a 60mm if you wanted no problem. The only thing you want to avoid is one that is too small. That shouldn't be a problem if you pick up a 44mm. Plus it'll cover you if you decide to upgrade your turbo later.

I do think the turbo is a little small though. The hx35 is only rated for something like 60 lbs/min (roughly 600 hp completely maxed out) so to make over 500 hp you'll be pushing it out of its efficiency zone (ie more boost pressure) and that means HEAT. I'd definitely recommend a good intercooler.

Also, the turbine is small too due to the fact that it's designed to run on an engine that never exceeds 4k rpm. On a gas engine this means that the turbo will spool quickly (a good thing), but will start to choke the engine at higher rpms. IMO, the hx35 is better suited for a 4 cylinder than a v8. It would work great as twins, but as a single not so much. If you're trying to keep it low budget there are other, better suited, low-cost options out there.