I'm on my fourth headset, and I've had it for over ten years and am delighted with it. It's a DC 13.4, with the gel earseals.
Last year I had it modified to ANR, and once I got used to the ANR (took a few hours - it was strange at first) I'd never go back.
So...
1. Very important on a long trip, hardly at all for local bimbles.
2. Absolutely. They fit fine round my glasses. The foam things are *horrible*.
3. I have the standard DC one and it's fine.
4. I think YES. Paid about £150 to have my headset converted. Everyone I know who's had the conversion loves it and would never go back.
5. If you want to, but they're all much of a muchness. Solid reliability is more important, and that's where DC scores. I bought one of those "cheaper but highly recommended" jobs a year or so before the DCs. I never wear them. Cheaper, but you get what you pay for IMO.
6. Probably, but never looked. I reckoned $250 new (mail order in the USA) wasn't too bad. Works out at around $25 a year if I scrap them now. But they're as good as new (better, cos ANR) so that's not likely.
7. Check the prices carefully - if you order from the USA for postage to the UK, you'll probably pay more than buying here (after duty and VAT and massive postage). If you're in the USA anyway, get them by mail order from a different state.
DC reliability is very good. I'm not sure, but you might even find the guarantee is honoured here. Never had to try.