I just ordered 3 mesh padded liners to wear under shorts or other clothes. You can pick up unpadded spandex shorts (if you're so inclined) just about anywhere, cheaper than at the bike stores and wear them over the liners. (Or, capris, shorts and other stuff - or whatever pants you already own).
www.teamestrogen.com has plus-sized liners and cycling clothing for us "less in shape" chicks, too... although it seems I'm something like a 3x when normally I'm a 1x elsewhere... *sigh* But it's a lot closer to my measurements than most of the bike clothes I've seen for what looks to be women the size of 11-year old girls.
But, after my long bike ride on Sunday and my sore butt yesterday, I'm ready to invest in some sort of padding. Ironically, my chiropractor says cycling seems to agree with me, and it's not knocking out my hip alignment or screwing up my back. (But I've been going to the gym and strengthening my erector-spinae muscles for a few months too). Aside from my leg muscles being sore, my shoulders were a bit sore as well as my abs. So, it's not just my legs getting a work-out!
Sandals vs. shoes - I'd go with shoes. Sandals on bikes aren't a great idea - usually people tell you that, but I can tell you from experience. When I was a kid, it suddenly started raining, and of course all the oil on the road washes off in the first couple of minutes... into the gutter. I was crossing a side-street, and down I went - skidded about 15 feet on my bare ankles. NOT a fun experience, nor the infections and recovery time it took after that. I'm just lucky I didn't break something, too. Basically - you can never predict an accident, but you'd like to be somewhat protected.
Mountain-biking has a lot of sticks, rocks, poison ivy and stuff. When I prepared for hiking Mt. Washington 3 years ago, everything I read said "get hiking boots, not sandals" and told me to wear long jeans, not shorts because it keeps you from getting scuffed up. Also, you probably want to wear some kind of socks so you don't get stinky sandal-feet, or blisters. It's not quite the same as a liesurely walk on the boardwalk.
Ultimately it's up to you, but I'd suggest erring on the side of safety and being a little bit hotter. (Then again, I'm used to wearing 3 layers of wool and a polyester powdered-wig in 96 degree summer heat playing my fife for fun... and rarely wear shorts).
I think I will look into some cycling shoes. I only slipped off the pedals a few times, but yeah - I think I'd prefer to share the burden with my hamstrings instead of just winding up with sore quads and foot-arches.
--Janis