I ride a D7, haven't tried other folders, but compared to my hybrid stye big wheel bike I find it a pleasure to ride. Furthest I've ridden it is 30 miles with a coffee stop half way. Absolutely fine, and I am only a casual rider. My hands/wrists did get a bit tired, so I've since added bar ends and that makes a big difference - a change of position, and allows me to stretch out just a bit more. For ref I'm 6'2, have the seatpost up to max (could do with it just slightly higher ideally) and handlebars about level with the seat. Gear wise, its not bad, I occasionally have to get off on very steep hills, but as I say I'm not super-fit.

I plan on replacing the saddle next, which I expect will improve comfort quite a bit - the stock one is not great.

Can't advise on whether P8 is worth the extra (I only paid £200 for my D7, and the P8 was almost twice the price, so it was an easy choice) but assuming you aren't super tall/short, you could certainly go the distances you're talking about on a D7, no problem once you've made a few tweaks.

Cheers, T