I,ve had 3 defender 90's over the last 15 years.
My current one is fairly modified truck cab thats lifted big tyres, Full roll cage etc. - All the gear no idea... with a 300tdi. Either tdi engine is great. Td5 is really good as long as electrics dont go wrong.
If your planning to use it offroad ie trialling or the like its amazing where a modified 90 will go. I,ve done a bit of trialling its great fun very cheap motorsport.
As said above happy around 55/60mph cruising even with my massive 255/85's mud tyres.
If you want modified try to find a good one where someone else has spent the money. Mine was 6k on a 98 plate with 180k on the clock but really well modified with all the kit.
I love mine, its rough & ready for anything and pretty damn cool. Even the missus loves driving it.
Lots of offroad work go for tdi or if your going to commute maybe go td5.
As said already. Look for rot on chassis everything else is bolt on.
Massive aftermarket for bits and mods. Great vehicles. Very slow but lots of fun.