Which makes the Druid effectively +10, since the system assumes that NADs are an average of 2 less than AC. This doesn't feel that horrible to me.Yeah, this group isn't really into power gaming. The group attack bonuses are:
Paladin: +9
Rogue: +10
Avenger: +11
Druid: +8
Although, as Ferghis pointed out, the Druid doesn't have any powers which attack AC.
A level 6 dragon would be EL+2 (+1 for being one level higher, +1 for being used against a party of 4), which is a hard fight on paper, but not overly so. Assuming that this is their first Solo, this seems reasonable enough. On average, they will be hitting on a 10+, which isn't too frustrating. A level 5 Solo with an additional monster (or minions) may be an alternative.
As for which dragon, I'm not sure, as I don't know them very well. Don't feel you are restricted to choices that are level 6, though, as it's fairly easy to adjust a monster up or down a couple of levels (HP by 8/16/32 for Standard/Elite/Solo; attack rolls, damage rolls, and defences by 1; skills and initiative by 0.5; DCs according to the table; everything else is pretty much the same). I would suggest sticking to dragons that are using the modern stat block, however, as Solo design principles changed at around the same time.