I have an H3+ in my Anderson and it's a good pickup, but I don't love it. I have no idea what the resistance is, but it seems like it's lower than I would expect for their highest gain pickup.
From the look of the direct mount pickup routs, other pickup brands *should* fit but I wouldn't want to state anything definitive. However, it's not like the curved leg routs on a Suhr, where you'd have to re-shape the pickup legs of a Duncan to fit.
When it comes to dealers, I just use Music Zoo and have nothing but good to say about them. Anderson are great to order from - they'll contact you either through the dealer or direct if there's anything they want to clarify.
As for the comparison with Suhr, I have my custom order Anderson classic and I have a Suhr Reb Beach, so I can't compare Angel to Modern. Mine are both great guitars, but I think the Anderson is the best guitar I own. The Wedgie neck join is a great, great thing and that is what gives it the edge over the Suhr to me. From a visual perspective, I've always thought the Modern looks weird with the way they've offset the body, whereas the Anderson is spot on.