based on what you have said,
with respect too you i think the next step for you is too spend 6 months selling some dearer cars from home without the overhead to get a feel, the reality is these are the cars you will be needing to sell to make any money, believe me there is a big difference on a monday morning to how much you "try" to earn this week and what you "have" to earn a week once committed to full overheads.
regards 50k, at your stage it is too much especially if you have borrowed it, when things get tuff you will go out and buy more with 50k this will not help you, try 20k across 3-4 cars from home, the biggest issue for everyone starting in this job is contacts and sourcing stock, more money is not the answer at this stage, if you do get established and are earning good profit then don't worry about needing more money, there will be plenty of people happy to give it you if your doing well.
and also, with 20k you will manage the 3-4 cars better and probably enjoy the job until taking the bigger plunge.
good luck.