well... depends on how you go about it... an engine rebuild with decent components can eat half of that in parts alone..
Engine wise and considering the condition you got it you should be checking for bore wear and potentially consider a rebore and new pistons... then rods which is a prerequisite of any 1.8t build..
Cranks are solid and strong, don't wear much but replace all shells... use ARP mains studs and plastigauge the journals to be sure
Stick to OE head bolts if only aiming for 400 hp, OE headgasket is the best choice..
Head wise you will need to get any head you get hold of checked for valve guide wear (particularly exhausts)... valves are another weak point of 1.8t's... supertech inconel triple groove exhaust valves are a solid replacement along with cat cams gold springs...
Inlets are a bit more resilient but supertechs are also a good choice if budget is there
New oil pump and pick up is advisable..
As for turbo setup choice... there are hybrid K04 turbos... useful as its essentially bolt on and off the shelf K04 upgrades all fit but generally speaking you won't get 400hp from them...
There is the K04-064 route but they are tricky (although not impossible) to fit in a quattro car but you will need to fabricate parts such as intakes, downpipes, coolant and oil lines...
Those are the relatively cheaper options... for more flexible options you are looking at turbos like Garretts...
The turbo cost varies a little but will be more expensive overall... the current setup favoured by Bill at Badger 5 is a tubular manifold, external gate and AET GT3076r (billet wheel)... this will essentially cost around 3k alone... and you will still need a downpipe and intake...
Other things of course are an inline fuel pump to support the extra fuelling, larger injectors, decent FMIC, decent exhaust (and not just a miltek as they are restrictive at this level of power)
Then there is the mapping
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