Welcome to the world of QRP.
About 7 years ago I was in the same boat: living in an apartment and wanting to get back on the air. I had decades of QRP experience so bought an FT-817ND to operate portable. I have never once regretted the decision. Once I moved into my present home I actually used the 817 daily for a year as my main rig, running CW, PSK and JT65a. It's got it's warts but as a mini-station-in-a-box it's hard to beat. The rig, the antenna and all the extras fit in a cheap over-the-shoulder bag.
I would recommend that you do some research in two areas: batteries and antennas. I went to the HFPack website (they're the folks that love operating such things as pedestrian and bicycle mobile) http://hfpack.com/antennas/shootoutvertical2002.html
and looked at their data on antennas: they've done "shootouts" for both vertical and horizontal lightweight/portable antennas. The data is worth a look. Based on their results I decided to buy a Super Antenna MP-1 and an additional 57" base rod (from Hustler). My particular reasons were: I didn't know that I'd always have decent trees around for wire support and this thing C clamps onto a picnic table. First day out I worked Greece at 2.5 W on 17 meters.
The battery issues have to do with the relatively high current needs of the rig. There's plenty written about what works. I have the newest W4RT 2700 mAh battery inside and it's good for several hours of QSOs or better than 4 hrs of listening. The basic 7 aH gel cell seems to be a popular option.
If you've already done QRP you know all the "you'll need more power" doesn't hold water. I hope you enjoy your new rig!
73 de WB0FDJ Doc