All my bike are road bikes, with my long distance commuter (long distance commute is roughly 44 miles round trip) being a mongrel road bike of mixed heritige, and the "short distance" (depends on the terrain, right now the commute I ride this bike on is ~24 miles round trip) commuter is a fixed gear Raleigh Rush Hour (geared 48x17).

That said, I am conservative with my money too. If I were in your shoes, I'd get some slicks for the mountain bike and ride your commute route for a couple months before shelling out for a new bike. I consider any (one way) distance less than 20 miles over reasonably flat terrain to be doable on any bike in the stable. I'd get the slicks though, rather than suffer with knobbies.

If you are intent on a road bike right now, get something that has clearance for fenders. The Trek Portland seems like a good bike, though I've never ridden it, it seems well thought out. Anything from Surly has clearance for fenders too. My long distance commuter bike frame is a mid 90's Schwinn road racing bike I got for $70 off eBay. It doesn't have as much clearance at the back wheel (you know how much it rains here) as I would like, but it is not as bad as modern road racing frames.

If I had it to do over again, I'd probably pick the Surly Pacer frameset. I've even thought of switching my long dist. commuter to that frameset to get better clearance in the rear wheel area, but couldn't justify the cost of replacing the perfectly good frame I have on it now. The Pacer is what I would consider the perfect road commuting frame. I like the handling of road bikes, as opposed to touring bikes, and the Pacer has the brake bridges in the front and back sized to accomodate long reach road caliper brakes, as opposed to the short reach standard that everyone uses now. It also has braze-ons for attaching fenders and rack; another feature which is left off most road bikes now-a-days.