I love your setup, you'll do fine.
Some things to keep in mind, if you do 10 gallon batches, you're right, you'll need a larger BK, 15 gal. As far as mashing in 10 gallons, personally, Ive never made a 10 gallon batch above 1.060 (since those tend to move slowly and I dont want 10 whole gallons of it) and have had no problem mashing in 10 gallons sub 1.060 in my 10gallon cooler.
You'll be great with the HLT cooler, personally when I started I went with HLT/BK in one as I would only do 1 batch per day, that worked out great. The HLT would be more awesome if you had the ability to control the temp in it instead of maintain temp. Have you given any thought to going electric with it? Even a 120v 1500w heatstick (http://www.homebrewtalk.com/f51/heatstick-awesome-142803/
) would be great. It'd give you the ability to raise and maintain your temp for your mash schedule.
Just my 2c, but your system is better than mine, and Ive been doing AG for 2 years now