I'd only buy the same version I knew worked for me online.. Shoe companies have a habit of messing around with shoes between versions, making it as likely that they'll work as not. Problem is if they don't, you won't know until it's too late to return them.
My local running shop does a 30 day returns policy if they recommend them to you following a bit of a run on the treadmill, worth the extra for me at least.
From what I remember of the course from a few years ago you should be OK with road shoes if you're happier in them, provided there's no substantial rain before the off. It's all well trodden paths, nothing loose or rocky
There's a mile or so of cattle trods which might get muddy, but then you just have to pick the best line through
Instead of looking at this as fitness training, think of it as efficiency training. While you may feel you are running easy @ 150bpm, imagine running at a similar pace with a hr @130 bpm. Then think how much faster you could run @ 150bpm if you had this efficiency.....
The SBU is a pretty straight forward route, with only 2 major climbs, Honister and Dent towards the end, with the remainder being mainly flat. By far the hardest bit of the course is the descent from Honister to Buttermere (I think).
When I ran it a few years back, the course was not marked - something to be aware of, if your nav. isn't that great - and you can spend a fair amount of time puzzling the roadbook instructions.
There are only a few sections which are tough underfoot as TRex describes, the remainder is a mix of fire road, footpath and some road.
Nice finish location at the School in St Bees and a free beer to top the day off, what more could you want?
As with all training, specificity is the key. You have two challenges -
Running 6 consecutive days and recovery.
Terrain and climate.
Your training needs to reflect these as best you can. I'd suggest building up to running 3-4 days on the trot regularly, with an occasional (once a month or less) week of 6 days running. The structure of these days is really up to you, but try and keep some quality in there.
Not sure what the terrain on the Coastal Challenge is, but try to simulate this as best you can. Same with climate - many layers on a warm summers day maybe?