First, I dont really know about touchscreen costs.
Second, keeping the first point in mind, you can easily do this for under 1k. Main point being, you dont need a beefy graphics card for an HTPC. Decoding HD content is mainly a processor intensive feat, but even that should not be a problem for any reasonably powered computer, especially if thats basically all you need it to do at one time.
Third, peicemeal will always be far cheaper. However, if you want something thats ready to go, and that you dont have to be the tech support guy on at work, then Id look into premade options.
That said if I were to build a HTPC today I would probably go with something like this...
Case -
http://www.newegg.com/Product/Product.aspx?Item=N82E16856101034Processor -
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115052Ram -
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231098HDD -
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152100BluRay (if needed) -
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106227If not Needed -
http://www.newegg.com/Product/Product.aspx?Item=N82E16827101131And if you need another Vid Card -
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500033Just as an example... that would run you...
$715 plus shipping for BluRay
$602 for just a dvd player
And it gets you a Core2Duo with 750 gig HDD, 2 gigs ram. You could definitely get by with a much weaker processor as well, and depending on your HDD needs, a smaller (and cheaper) HDD. You probably dont need 2 gigs ram either, but with it being so cheap, Id aim high!
On board sound should be fine, as should just about any video card (including on board). I chose a shuttle just because I like em!
Edit: Also, if you are all Mac users, perhaps going along the lines of a linux based machine would be good as well. I know there are dedicated
HTPC linux based solutions out there, then you could also cut your costs by not needing to purchase an operating system. I have no experience what so ever in interacting Linux with Macs over networks though, but I assume there are solutions.