I build wheelchairs too! 

After years of testing and trying all that was available I decided I could build a better one from the best one available with some serious re-engineering to suit my needs and to drive a car from.

Most power wheelchairs are awful things. Its not hard to improve on them!
See this powerchair:

These are also my all day power wheelchairs as well. I wish the manufacturers built powerchairs like this then I wouldn't have to do so...  

A Modified Powerchair page 

Important!  Don't buy a vehicle first! This is the biggest and most costly mistake people make. If you already have one then its probably best to sell it.  You are unlikely to be able to modify it in the ways you want or expect unless its the correct vehicle, and its nearly new. .

The majority of adapted wheelchair vehicles are adapted from brand new by various companies They only do certain modifications to certain vehicles.  If you do buy a non adapted car or van first then it is going to be an expensive mistake in more ways than one. Vehicles supplied already adapted for disabled usage are VAT free! That means you get a further 17.5 percent off here in the UK. (Updated -- now 15 percent)

Different vehicles lend themselves to be adapted in a variety of different ways.  If a dealer has developed a regular modification to say the likes of my new van above by Rollx, it does not mean he can do the same thing with any other vehicle. These ramp vans are based on the Chrysler minivans, Toyota Sienna and one or two others are built almost on a production line basis,

In the case of the Braun Entervan and Rollx Adapted vans, VMI ramp vans for example, a very special and carefully designed and manufactured adaptation has evolved over the years.  It is pretty damned major!  It involves such things as specially moulded body skirts, suspension mods, a complete new manufactured floor, Different designed exhaust systems, various carpets and interior plastic mouldings and seat mountings etc and much more. These parts are manufactured in quantity to modify only this one vehicle. These parts will not fit any other van.  So each company will supply only their conversion based on their development and history.

Before you visit a mobility dealer visit as many other web sites as you can that offer the kind of information you need! Read and research a LOT!  When you have narrowed down your search and have an idea of what you want allow at least 3 or 4 dealers to demonstrate their vehicles and modifications and advise you of the pros and cons. But remember they are selling you a vehicle! Read between the lines and ask as many awkward questions as you can! Don't rush. Don't buy the first "solution" you see. Find out and learn as much as you possibly can and think about it first. Mistakes are costly!

• Would I be ever be able to drive a car again?  Answer yes!  The driving part is easy. An automatic gearbox and hand controls are all that's required. Its the getting in and out part that I struggled with!  I am too fat and its hard work.  There are various solutions if you struggle to transfer - see "turney seat" picture below if you must use a normal car!  This can be done on your own too with a manual chair because some wheelchairs are collapsible and can be folded and dismantled to store in the passenger seat next to you. Its slow and hard work though if like me you are paralysed from the chest down.
   

• would I be ALLOWED to drive? Yes.  No problem there then! Just tell the driving licence authorities.
   

• would I need a different licence or test?  No, if you already had a licence nothing changed! If you have not got one yet then you need lessons and a test just like everyone else. There are special schools and instruction for this. I will add some links here later.
   

• How and what modifications would be needed? Very little in my own case initially since my existing Automatic Ford Granada just needed some simple push pull mechanical hand controls fitting at around 160 UK pounds.. Cheap!  Takes about an hour to fit.  If your car is manual you will either need to change it or get a fancy Simplex Guido clutch installed in your manual car which works like a twist and go moped. But that's expensive and autos are better anyway. Who in their right mind actually wants to fight a clutch pedal and a gear stick every thirty yards in our traffic...