HSTs, 37s and 47s will all be fine, as they are 1960s designs which are still in service today - as long as the route can support them of course.
Actually, considering 2004 is for all practical purposes modern era, you'd be hard pressed to find anything too new to be prototypical. Finding stuff that is too old is more likely. So don't bother putting in steam locos (unless they are preserved machines) or a 55 (likewise) or Mk1 carriages or the old RCH-style wagons (easily identified by being much smaller than modern wagons). Indeed the original Mk2 carriages in the default stock aren't seen much any more either.
Note that 166s are unlikely, because they are built specifically for the extra-wide loading gauge on the GWML, so *cannot* run outside former GWR broad-gauge territory.
Also beware of using foreign stock, as Kuju didn't bother to separate UK and German wagons in the asset categories. If you are unsure, have a look at this scenario, in which I carefully separated stock into three areas of the yard for US, German and UK:
 | |  | Default Stock Roster [54762 bytes] - default_stock_roster.rwp File ID: 26866 Date: 13 Jul 2011 - 228 Downloads |
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