I'll stick my 2p in as ive just finished my build.
First of all, Nothing wrong with Ebay turbo's so long as they are sent away before installation for a 360 degree thrust bearing upgrade and new seals. This is what stealth do on their kits. You want to be looking for one around the 0.82 A/R mark for street use.
Injectors, DO NOT get the cheap ones on ebay/amazon. I had 6 delivered, 2 stuck fully open, 2 didnt fire at all and 1 fired intermittently and only 1 fired as normal. Ended up hydrolocking the engine at idle with fuel. Drained 10L of fuel from the sump. Emerald sell genuine Bosch 440cc injectors suitable for a VR6 at around £60-65 each.
Wastegate wise, i am guilty of using a cheap wastegate but that's due to be changed very soon. Heard bad things about them but im about to change the spring so i'll inspect it then. I'd go tial or turbosmart as previously mentioned.
Install ARP head and rod bolts when you do the spacer. Use a mk4 gasket, seperate the 3 layers and use copper spray to seal the two outside parts either side of the spacer IIRC.
MAF use a 4" housing and a genuine bosch Maf sensor. Dont use anything else if you plan on going for a 440cc chip or remap.
You'll need upwards of £3k to do this well. Anything under that expect problems.