I'm going to look just at purchase price. Essentially, you can't always claim the whole of the purchase price (or 95% your case) in the year (the accounting period) of purchase, but you get a percentage of the value of the car each year, called writing down allowance, which is a capital allowance. It is similar to depreciation, but based on HRMC's own formula. In fact, it seems you probably can claim 95% of the purchase price, because the value is less than £1000. The logic is a bit involved, but I hope you can understand it.
You could also claim simplified expenses instead, which is just based on a rate per mile, but you can't claim both.
Note, by year I mean whatever your account period is. This could be the normal financial year, but you would probably have a better idea about this. See The HMRC webpage on this for more details.
The Basic Idea
The big idea is that you record the value of any assets you are claiming writing down allowance on in one of a number of pools, that attract the same rate of writing down allowance, so you don't need to record the value of each asset separately. They are similar to accounts in accounting, so they have an opening balance, and closing balance. If you use an asset for personal use, it needs a pool to itself. HRMC call that a single asset pool.
Finding your Rate
So, to start with, look at the Business Cars section, and look at the Rates for Cars section, to determine the rate you can claim. Each one links to a further article, which gives more detail if you need it. Your car is almost certainly in the special rate category. Special rate is 8% a year, main rate is 18%, and First year allowance is essentially 100%.
How much do I claim?
Then, you look at the Work out what you can claim article. That talks you through the steps. I'll go through your example. You would have a pool for your car, which would end the account period before you bought the vehicle at zero (step 1). You then add the value of the car in the period you bought it (Step 2). You would reduce the value of the pool if you dispose of it in the same year (Step 3). Because the car is worth less than £1,000 (see the section on "If you have £1,000 or less in your pool"), you would normally be able to claim the whole value of the pool (the value of the car) in the first accounting period, and reduce the value of the pool to zero. As you use the car for personal use, you only claim 95% of the value, but still reduce the pool to zero. See the section on "Items you use outside your business". This £1000 is adjusted if your accounting period lasts more or less than 12 months.
Once the pool is down to zero that it you don't need to think about it any more for tax purposes, apart from if you are claiming other motoring expenses, or if you sell it.
If the car is more expensive
It gets more complicated if the car is more expensive. I'll go through an example for a car worth £2,000. Then, after Step 3, on the year of purchase, you would reduce the value of the pool by 8%, and claim 95% of the reduction. This would be a 160 reduction, and 95%*160 = 152 claim, leaving the value of 1860 in the pool. You then follow the same steps for the next year, start with 1840 in the pool, reduce the value by 8%, then claim 95% of the reduction. This continues until you sell or dispose of the car (Step 3), or the value of the pool is 1000 or less, then you claim all of it in that year.
If you sell the car
Selling the car, or disposing of the car is discussed in the Capital allowances when you sell an asset article. The basic idea is that if you have already reduced the value of the pool to zero, the price you sell the car for is added you your profits for that year (See "If you originally claimed 100% of the item"), if you still have anything in the pool, you reduce the value of the pool by the sale value, and if it reduces to below zero (to -£200, say), you add that amount (£200, in this case), to your profits. If the value is above zero, you keep applying writing down allowances.
In your case, that seems to just means if you sell the car in the same year you buy it, you claim the difference (or 95% of it) as writing down allowance, and if you do it later, you claim the purchase price in the year of purchase, and add 95% of the sale price to your profits in the year you sell it.
I'm a bit unclear about starting "to use it outside your business", which doesn't seem to apply if you use it outside the business to start with.
Simplified expenses
You can claim simplified expenses for vehicles, if you are a sole trader or partner, but not if you claim capital allowances (such as writing down allowances) on them, or you include a separate expense in your accounts for motoring expenses. It's a flat rate of 45p a mile for the first 10,000 miles, and 25p per mile after that, for cars, and 24p a mile for motorcycles. See the HRMC page on Simplifed Mileage expenses for details. For any vehicle you decide to either claim capital allowances claim running costs separately, or claim simplified mileage expenses, and "Once you use the flat rates for a vehicle, you must continue to do so as long as you use that vehicle for your business.you have to stick with that decision for that vehicle".
Summary
In your case, it seems you can claim 95% of the purchase price in the accounting period you buy it, and if you sell it you add 95% of the sale price to your profits in that accounting period. It gets more complicated if you have a car worth more than £1000, adjusted for the length of the accounting period.
Also, if you change how you use it, consult the page on selling selling an asset, as you may have disposed of it.
You can also use simplified mileage expenses, but then you can't claim capital allowances, or claim running costs separately for that car.
I hope that makes sense, please comment if not, and I'll try to adjust the explanation.
