When I had an allowance it was £4,800 a year and that was on a £40K a year job. What you can and can't drive is entirely up to the company. For us they used to impose the same rules on models of car as they did for the company cars so no soft tops for example. They also stated the car had to be under 5 years old, but this tended to be flexible. If you drove a prestige marque then they realy didn't bother wit the age thing, provded it wasn't a 20 year old heap of junk.
I never quite understand the rules about claiming mileage back but cretainly we were restricted to 14 pence a mile like Rotty. Mileage rates are all about compensating you not only for the fuel but wear and tear etc. If you had a company car then the wear and tear element is irrelevant, hence such a low figure and they just keep the same number if you take the allowance instead it seems.