I did CTS Dorset mara last year. Weather was perfect, very cold so ground was firm and clear skies. The course is a brute. The hills around Lulworth go straight up and then come straight back down. I would recommend the marathon but expect it to take quite a bit of time.
When signing up I was toying between mara and ultra, was very glad I chose the marathon, just because the Ultra route sends you back around the half mara course (actually about 16m) and then the 10k just for laughs, so you end up running some bits 3 times.
At about £50 I thought it was very expensive, and I went back to run some of the path again a couple of weeks later and *all* of the course markers on the 2nd half of the route were still out. Very poor and enough for me not to want to use Endurance Life again. If they're willing to accept the best part of £50k in race entries the least they could do is clean up properly afterwards.
The latter end of the Ridgeway is a doddle to navigate, just make sure you know how to join up the two ends of trail either side of the M4. Lots of potential to second guess yourself there as the stretch up and out of the village is much further than it looks on a map. Finding your way through the village after you cross a main road about 5m later too I suppose if you've not been there before.
For me night running really brings it all home. You're generally quite worn down by the time it gets dark and can start to experience some pretty surreal moments.
Trust your training too, don't give in to the temptation to push it and do too much this late on. I did exactly this in Jan and am still not back running. I did sneak in the TP100 in May though, mostly walking, no prior training, goes to show how much of it is in the head.
Nathan do a handheld, it's changed a bit since I got mine a few years back (mine is quickdraw elite) but it's probably the best bit of running kit I've ever bought. I tried the Ultimate Direction ones in a shop and they felt fine but the Nathan one easily trumped it. Added bonus is it gives your hand a bit of protection whever you fall over. I can carry that thing for 24 hours and still not really know it's there.
I sometimes attach bottles to the straps of my rucksack and while that's ok, it's much less practical than just having the bottle in your hand ready - I always find I drink less and get dehydrated more quickly. That said, if you can eliminate the bounce then it's workable for races, but in training I'd generally put spare bottles in a pack and stop to refill the handheld. Also... unless it's really hot I'll often be able to survive on the handheld between checkpoints, so no need for the front bottles. That one depends how much water you get through though, I know I drink less than most. If you're in a race and want to use poles then the handheld is not an option, so that's something else to factor in depending on how much spare cash you have and your running goals.
You will get given a bus ticket with a time on when you register to help prevent everybody arriving at the same time. I doubt it matters too much if you're not there at that exact time but it will get busy.
You can always take a portable USB charger and charge up for an hour or so half way through. That said I don't think I'd bother for this race, knowing how far to the top would probably demoralise rather than incentivise! On one climb I remember thinking I must be about to summit for a good hour before I actually made it to the top.