what amp and speakers for this amp?

I'm trying to build a super simple system on a budget. I found this amp used http://www.crutchfield.com/S-qk07uNuqOVG/p_500MRPF240/Alpine-MRP-F240.html . Can I run a sub and fronts off of that? It doesn't need to be super loud or anything. If that amp would work, what specs should I look for in a sub and front speakers so that it all matches properly? Thanks

Yes you can run your Fronts and your Sub on that amp. You will need a subwoofer that is one of the following. 4 Ohm single voice coil. 8 Ohm Dual Voice Coil. or 2 Ohm Dual Voice Coil. Either of the 3 above mentioned woofers will allow you to get the maximum power out of the half of the amp that is connected to the woofer. The wrong impedance woofer will not allow you to get maximum power, with this amp, you will probably want to get the most out of it that you can.

Thanks for the info. What kind of power range for the fronts and sub should I be looking for?

With that amp, you should be more concerned with the efficiency rating of the speakers.  The rating you are to be concerned with is the XX/DB at 1 watt at 1 meter.  The higher the number the better the speaker is gong to be for your system.