itsamacthing,
Moving to a Mac network from a PC one is not impossible and we help companies do it all the time. I was intrigued by your reference to "thinstuff terminal server" and I'll have to check into that for myself.
Regarding QuickBooks, we have had very little luck in switching people over from QuickBooks on PC to QuickBooks for Mac. Without exception, our customers were unhappy with the migration and so we began offering a Parallels or Terminal Server solution for them. However, nowadays, when customers ask us about QuickBooks migration, we steer them towards Account Edge. So far, all the customers we've moved from QuickBooks on PC to Account Edge for Mac have been pretty happy. There is some migration headaches to overcome and a little bit of a learning curve, but once the transition is complete, the Account Edge system works quite well and gives our customers all the features they were looking for in QuickBooks. You might want to check into it and see if it's a fit for you.
Regarding collaboration via Exchange... Mac OS X Server's collaboration suite isn't bad. We use the calendar, contacts, and email system of Mac OS X Server extensively and can recommend it in most cases. If you're 100% Mac, it is even easier. Here's what I can tell you...
Mail works GREAT. I have no problems with the email system on Mac OS X Server. Solid as a rock and easy to move if you need to. Works find with PC email clients like Outlook or, better yet, Thunderbird. Lion Server also has a new webmail engine that is pretty snazzy.
Calendar is OK. Had some trouble with invites to individuals on other systems, but besides that it works well. I could easily work with and move the data store on Snow Leopard server, but in the new Lion system, I cannot figure out where the data is stored or how to work with it. Accessing calendars via Outlook on PC isn't very good, but I understand working with Thunderbird works well (calendar clients must support CalDAV). Lion Server also has a webcal engine for working with calendars through a browser that works pretty well.
Address Book is OK. If your address book is really large (10,000 contacts or so), it may not work well for you. It had some MAJOR problems in Snow Leopard (1000 contacts would slow the address book client down to a crippling crawl during updates), but that has been improved and maybe fixed in Lion. As far as I know, there is no way to work with the Address Book if you're on a PC, so keep that in mind. No way to look at contacts through a browser either.
Anyway, I think this message is long winded enough. I hope this answers some of your questions!
Alex