There are a fair number of differences between the earlier turbos and the later (almost last of the line) 4 cylinder turbos.
The engine you have appears to be a good starting point (in that it’s much later etc) that someone has swapped the earlier manifolds etc onto.
I can’t see from your pics if it has a charge cooler?
I can see that it has the earlier (as fitted to 86 cars for example) exhaust manifold, turbo and wastegate setup - S4, S4s, S300 had a different manifold and turbo configuration along with the charge cooler and fuel injection - the charge cooler and fuel injection also results in a different intake setup which all contribute towards the higher horsepower figures considerably.
There where carb turbo cars with charge coolers - this may be a sensible route for you if you are looking to make good use of your later engine but there are a few parts that you would have to find/change in order to do it. - as you have already mentioned, the Citroen box is likely to be your weak link as the power increases. - even the Renault box in the V8 was only rated to 350bhp (pushing its limits) but if you are careful it can handle more.
There are many different things to look out for if you are rebuilding the engine, it’s a bit of a minefield tbh as parts have changed over the years (different suppliers = slight differences in specs etc). Even head gaskets used to be different thicknesses between the LC and HC - it may have changed now but I recall a revision to one gasket fits all some time back which to most wouldn’t mean very much but effectively made a HC into an LC and the LC into an XLC….!
The standard car is no slouch, I’d just put it together and enjoy it as it is.
