I can categorically tell you that starting work will not affect you having a Motability vehicle. You pay for this with the mobility (higher rate) component of your DLA which is paid irrespective of whether or not you are in work. If you are lucky enough to find suitable work (never say never but do bear in mind that plenty of fit and able-bodied folk are struggling to get work out there at the moment) you may be able to claim Tax Credits if you work in excess of 16 hours/week.
Would advise going through the DEA (Disability Employment Advisor) at your local Jobcentre (although the quality of these people seem to vary vastly). You could volunteer for something called Work Choice which is voluntary programme to help disabled people into, or back into work. You may also like to consider trying out some voluntary work to see how you might realistically cope with a paid job; it would also help boost your CV which won't do any harm in the current climate.
If you do manage to get paid work then seek a workplace assessment from Access to Work; again the DEA can point you in the right direction. Sadly, as part of the Government cuts, AtW's role has been greatly eroded meaning that employers are now responsible for financing many of the reasonable adjustments that AtW used to fund (another practical reason why disabled people are finding it even harder to get jobs) including potentially costly alterations to buildings. However, it's still worth it so you (and your employer) can see what's what and AtW will still cough up for a lot of "specialist equipment" although it has to go higher up the chain for a decision.
Finally, I would also advise getting a copy of the MS Society free booklet on working with MS (MS Trust do one as well). There's also one called "Worried and Working with MS" (or similar) which establishes an open dialogue between employee and employer with regard to managing the condition. I would really recommend following the suggestions here and, if the workplace has one, join the union. Sadly, I have been bitten twice now by employers who should have known better.
The first situation was resolved but not without an employment tribunal claim for failure to provide reasonable adjustments. I went on to give them two good years without a day's sick leave before I left to take up a "fantastic" post - with a disability charity no less - managing a training programme to help disabled people into work. Although I got all the environmental adjustments, I did not get the flexible working I was promised before I took up the post and, to cut a long story short, it all went horribly, horribly wrong and, after only 6 months in the job, I had no recourse through the legal system. Bear in mind that if you don't complain about problems straight away (and by complain I mean formally through a grievance) then you lose any right to do anything about it after 3 months - and, let's face it, who wants to start "making a fuss" during what is likely to be one's probationary period.