I'm going to disagree with most of the posters here. Why risk damaging a good carbon frame by spending a bunch on various rack adapters to kludge something together.

Unless you have an ultra-light laptop, you definitely want a rack and panniers. A standard laptop is typically ~8lbs with another 2-3lbs for the charger. 10lbs total isn't bad for a messenger bag or backpack but if you're also carrying clothes/lunch you can easily hit 20+ lbs. None of the seatposts bags mentioned have a prayer of fitting a standard HP laptop, just totally the wrong shape.

From a budget perspective, you're probably looking at Al frames on any new bike. Steel is a good option, but its heavy and not cheap for a non-Walmart bike. REI has a good selection of steel/touring bikes at decent prices. This Novara has everything you'd want for $800 Novara Verita Bike - 2014 - REI.com. It's an extra bargain if you have an REI dividend to use.