Much (if not everything) will depend upon what you want to establish. As you say, your requirements are loosely defined, so I would suggest to tighten them just a little bit more.

If it comes down to learning how to program a 805x, even the simplest (read: cheapest) board will most likely do. Flash-type ISP processor based boards in my view prevail over the ones that are available with a small 'bootloaders' to get your program up and running. In general, when you want to expand the possibilities of these boards, you'll have to grab your soldering iron and make yourself whatever you need. Advantage: you're in control. Disadvantage: you're in control.

The PJRC board you indicate looks fine to me and seems worth its price. It has all the stuff that I could really think of you would need to get onto that learning curve. Bear in mind that at some point you may find that all these nifty little extra's that come of use when you're down on the ladder may become a block on your feet. To postpone that moment as long as possible, it is quite important that you can disable built-in software-features (such as the monitor that is included) that prevent you from having full control over the micro. As for the PJRC, it seems to me that it offers that possibility, but I must add that I've not used the board myself so I base that only on the information provided on the web-site.