Firstly - go for it. Secondly - good luck.
Lot of stuff already covered. I ( in extremely small way ) gave a hand to Simon King a few years back ( NO not that Simon King ), he did Sussex to Lands End then John O Groats with a pony and wagon - see Simon King Return to the Land - via Google or Allied Special Forces Association webpage.
Simon overcame a great deal of negatives - but that was part of the challenge. Think that things got rather complex toward the end and he was glad to be done and get back to a day job. But nothing detracts from the achievement and I hope he can look back with considerable pride - both for what he did and more importantly how he helped/ affected others along the way.
Charity wise it can be fraught - because lots of people try that route. But everything helps. Do it for you first and if that generates some cash or publicity for a good cause then great. The A9 does tend to be littered with walkers, bikers etc on this run between June and September - so what!
It is trying that counts. Strange things happen when you step out of the grey of everyday life - a lot of it very good.
I am fortunate to split time between Glasgow and Forres and have good contacts more or less the route from Crawford up to Inverness. Happy to help - ended up giving Simon my house for two days!! If you would like to talk with Simon I can make contact and check how is is doing - let me know via PM.
Simon had neither walked nor worked horses before the trip - he learned fast! By the latter weeks his pace was incredible. ONly downside - he didnt want to leave Scotland
Safe journey