if you're flying into Prestwick, I would recommend missing out Glentress and drive up the West Coast to Glencoe/Fort William. I think you can hire bikes in Alpine Bikes in Glasgow- if not you will be able to hire them in Off-Beat bikes in Fort William.
Fantastic riding up there with a good mixture of natural and man made trails which easliy beats anything Glentress has to offer. From there up to Torridon for a couple of days and then over to Aviemore.
Aviemore has reasonable bike shops and the folks will be able to advise of good routes.
Kenny Wilson's book Mountain Bike Scotland is a good starting point. (although I've found some of the routes to be painful MXCT)
It's worth posting on here when you've got a rough idea of where you're going to visit.
PS Get some midgie protection.