I was thinking of grabbing some lowers while I save up for the all the other parts of an AR. I don't have much any experience with them so I thought I'd ask you.

Are there any differences between AR-15 lower receivers?
Are there any brands that you would not buy, or recommend?
Anything else you can think of?

If you stick with any major brand you will be fine.  Ofcourse the top of the hill choices would be a LMT or Noveske.  I have two CMMG lowers that have served me well, a friend of mine has a Sabre Defense.  The only lowers Id stay away from would be Olympic Arms and some first gen Smith and Wesson M&P lowers.  Between the two, a lot of the lowers were out of spec, some not accepting certain mags, others have poor tolerances.  I tried to look for this chart on M4carbine.net showing what factory puts out each brand of lower.  You could look on ar15.com and find this as well.

When it comes to building an AR your lower parts kit and bolt carrier group are the most important.  Next of importance would be your barrel.  Stay away from DPMS lower parts kits, but if thats all you can find it will do.  As far as a bolt carrier group BCM or LMT from Bravocompany is good.