The 1.8 -> 1.8a Migration CD is just an upgrade, as far as I remember - only MetaFrame XP FR/SP2 is the full CD. You'll need to first install 1.8, then perform the upgrade, if I'm correct (it's been a while!). If it's a new server, that shouldn't be too much of a chore. The main problem will be that you only have 35 days to sort out any issues, if you only have 1 base license.
You shouldn't get any MetaFrame issues to speak of, IME, but if you're not used to Windows 2000 Terminal Services, you'll probably get a bit of fire-fighting to do. Best advice I can give is to read up on W2k TS as much as you can and get to know all the differences. MetaFrame will behave exactly as it did on NT4 TSE, so you shouldn't have any real worries there.
Gotchas include system profiles - W2k stores them in %systemroot%\Documents and Settings. This may affect your policies. Also, watch out for W2k licensing - it's caught most of us out at some stage.
Good luck with your migration! CitrixEngineer@yahoo.co.uk