I think
@JohnQ told me a while ago... "buy once, cry once"
I bought the 8 gal pot, then upgraded to 15. Even if you don't "need" it, it won't hurt. It's real easy to max that pot out on a 5 gal batch. If you figure a gallon of boil off, and .5 gal for trub losses during fermentation, you're starting your boil at 6.5 gal.
Plus, if you want to give BIAB a try it's a real easy way to get into all grain, and the 15 gal is a good volume for that!
This kit from Everwood is $139 and seems to be the best deal on the internet right now.
http://www.everwoodavebrewshop.com/product-p/ebs-pot-15g-ec.htmWith all that being said- you can definitely work with an 8 gallon pot, especially if you get a good deal for it.