The main things to get right with a McHale pack are the capacity and the rucksack part. I have a *huge* McHale pack (I need space for my monorail), with two compartments (the divider is removable), for example... that's the one part you can't really change once you've bought it. The material choice at that point is similarly critical -- if you can afford it, go for Dyneema; the weight savings was startling; my actual pack is bigger than the demo that Dan had me try out, but also significantly lighter.
I got mine with a daisy chain, and I'm glad that I did; I use that to attach things like my trekking poles when I'm not using them. That's something you'd want to decide up front, since adding them yourself would probably be a lot of work. :)
Most of the other accessories you can add later, like extra pouches and that sort of thing. Dan's interested in making sure that he makes the pack that *you* need, so he doesn't mind questions, and if you tell him what you're after, he'll advise you honestly.
I really didn't know what I would need when I got my pack, so I took what I was using at the time (a well-made but poorly-fitted Kinesis pack which was too small for backpacking with a 4x5) with my camera gear and said, "I need to carry this in addition to my backpacking gear."
I ended up trying out two demo packs; one was a panel loader which I didn't really like all that much, the other was a single-compartment top-loader, which I ended up preferring, even though without side access I couldn't get to any of my camping gear without extracting my entire camera kit. The lid is also an optional add-on, and you can swap it out if you want to.
If you go for a larger pack, I recommend the bayonet dealie. I don't use it much because of the camera, but if ever you decide to base camp, the bayonet feature allows you to turn even my huge pack into a moderate-sized day pack (that I'd estimate to be slightly bigger than a Windrider) with a stellar and custom-fitted suspension :)