Well when it comes to LR ownership, within a short time u'll endup being a enthusiast, fan, expert, mechanic, electrician,DIYist, "divorced" and whatever you may call it..
Anyways I feel you are not on a clear impression on the whole thing..
Back in the 70's and until 86, old land rover series 3s came in, these had a leaf sprung suspension and crude dash, 2.25L diesel + most body panels were different, However they can be fiddled with to make a defender look alike
these came in no.s like 28 SRI, 31 SRI, etc some even came as 32
the LAND ROVER 90s (not Defender) started off in 84, and the 90's came out in 86, these had coil springs, 2.5l diesel (12J), 5fwd box, AWD,and
panels are the same as a defender,
In 1991, they were renamed "defender" 90 or 110, to avoid confusion with other newer land rover models
Then they kept on upgrading the trim, engines(200tdi onwards) etc till now,
So what you need to worry is not about it being a Defender 90 or a LR90
just make sure its not a crappy convert(Frankenstein) of a Series 3, with some other reg book etc..
But mind you in most of the LR90 reg books from the auctions its mentioned as either LAND ROVER simply
or LAND ROVER 90/110
But you can confirm this by the chassis no, starts with SALLD XXX XXX 123456
the LD is seen only for 90/110 and defenders..
In terms of performance I feel there is very little difference between a souped up 90 and a real defender 90, because either way they are noisy, big, and poorly trimmed, except for the ones from YOM2007 onwards..