If you are planning on taking the plate off, get those forms in the post now as it currently is taking 2 weeks. I know as i did mine a few weeks back.
The problem you have though, is that you need to change the address on the v5 first which will take 2 weeks or so, then put the plate on retention = another 2 weeks. So your looking at 4 weeks at the quickest before you`ll be in a position for someone to drive away with the car, with the value of the car dropping every month.
Id definitely look at returning it to standard if possible as like someone else on here said, the money your looking for can get someone a minter 313 HR zed. So anything you can do to bring the price down to make it more appealing to Joe Public is definitely a plus, as at the end of the day Joe Public will be comparing this against other 2004 cars which maybe cheaper.
Still looking for these minter 313s online and really not finding them though...something of a forum myth I think people are coming around to realising - when on autotrader I found 22 313 cars and 3 of them less than £10k, average price of 313s on most sites is circa £13 and people looking for these on advice from the forum are not finding them as per some recent threads.
Understand my car is more than other 2004 cars, effectively though they are getting the upgrades at less than half used price, so if they have plans to mod its a good buy, if not, then they move on which is fair enough.