If you are thinking of what sounds like a (for their previous levels of spend) major outlay, keep in mind that SBS7 (now retitiled SBS 2011) is out soon too. If they might want or would actually benefit form any of its features (including some good remote working support last time I looked).
Eitherway, a second server (simple old hardware plus the new one) could also make a lot of sense if you are going to do a domain model, as it lets you have DCs on two different physical boxes and do some cross-back-up etc.
Having been in a similar position of unofficially remotely supporting family businesses, you are on a balancing act the more you lock things down the longer they work without assistance VS the more offen you are needed to let them do things they want to do as a result of the lock down.