I think that if you buy an used car ... the previous owner already paid the major part of the depreciation. Most cars depreciate most during the 1st year.
Therefore, getting a Lexus won't be that expensive, since the resale value already taken the depreciation into account.
Another thing (remember ... I am not in the UK, so I am trying to use this as an example, data is from www.autoexpress.co.uk)
USED CAR PRICES
Model Ins Grp 2002/51 2001X 2000W 1999T 1998R
LEXUS IS200 SE 12 (02)13350 (01)11700 (00)10550 (99)9110
LEXUS GS300 SE auto/auto 17 (02)16700 (01)14450 (00)13000 (99)11000 (98)9320
BMW New 3-Series 318i4dr SE 12 (01)11500 (00)10350 (99)8870
BMW New 3-Series 320d 4dr SE 12 (01)12450 (00)11250 (99)9580
BMW New 3-Series 325i/323i 4dr SE 15 (02)16400 (01)13750 (00)11450 (99)9790
If you look at these prices, then you see that the older the car, the closer the prices are.
The bigger the car (e.g. GS vs IS) or the bigger the engine (318i vs 325i), the bigger the depreciation.
The used value of the IS200 vs the 318i is pretty in-sync.
The used value of the 323i and the GS300 takes a huge hit during the first years, and depreciation of the 323i is much more than that of the IS200.
Using this info as base of my analysis, I do not think that the resale value of the IS200 will be significantly less than the 318i resale value.