It is a bold move giving up a position where you have a regular income stream to move into the self employed envirnoment where probably for the first 6 to 12 months, at least, no profit will be seen and you still have to eat and pay your other outgoings, but as you say unless you give it a try you will always be thinking "what if".
With that in mind I feel you need a reasonable bank balance to see you through these initial stages and be prepared to work 25 hour days to get things up and running.
Not knowing your circumstances/ abilities or running a buisness capabilities makes it difficult to offer explicit advice but be prepared for some big item expenses when starting up and do not forget when buying and selling income can be erratic and do not forget getting a good reputation is everything so you have to hit the ground running with a good quality product.
Yes there will be a raft of things you will never have thought of, plenty of things out there to trip you up, but if you are serious you will work through these OK and come out the far side smiling, hopefully.
I have been trading , quite modestly, now for lots of years and I do not think £100K would cover the replacement costs of the equipment I have, and there are lots of things I do not have.
Regarding what vehicles to to work with, your choice is endless but it is maybe worthwhile selecting a model that is a fairly good seller and sticking with them, becoming experienced and familiar with them, this experience will help when you come to buy/ sell stock, you need a regular income.
I have found in recent years it has become increasingly difficult to locate trade vehicles at a decent price where you can refurbish and turn a profit, I blame all the media platforms where everyone thinks their old wreck is worth a fortune, there is little good in spending lots of hours on a vehicle for next to no profit, so be careful what you select, check the auction sites to see whats what.
I wish you all the best if you do go ahead with this venture, everyone has to start somewhere.