A dealer won't care about the upgrades. Similarly a lot of dealers won't look to see if the cambelt's been done. These are things that might appeal to a private buyer though.
As an 04 does it have the higher CO2 engine? Again a dealer won't care (or probably even know) but a private buyer might seek out a car within the lower tax bracket.
If everything's in its favour I'd have thought £4500 - £5000 privately depending on how much further from Christmas you can get and how the economy fares.
To a dealer as a p/x or buy-in the quote you've had is about what I'd expect.
For another ballpark, get a quote from webuyanycar.com In my experience their online quote is about half the trade-in you'd get from a dealer that wants it, and 3-4 times less than what you might get privately.
As a guide I paid £3495 for a standard 2004 '54 GT JTD with 144k miles and FSH, fresh cambelt and waterpump and four good tyres.
2004 '54 GT JTD
1984 Alpina B9 (my other love)