My old mk2 RS2000 is out there still. I found it on eBay a few years ago, £30k and needing a full body restoration. Not anywhere near as good as my HPE either BitD.
RS1800 shells were pretty similar to the other RS's (a few extra strengthening bits here and there). In base form it was like a flat front base RS2000/Mexico but with a BDA. Of course they never stayed std for long on a whole.