Be honest about the car, but tell them everything good about it! If I'm looking at buying a car I want to deal with someone who's upfront about any foibles, faults or upcoming repairs/maintenance that it'll need - the car should be priced accordingly. If it isn't, and I spot something untoward which the seller hasn't mentioned and should really be aware of, I'm much more likely to walk away. However, there's no need to undersell it either. You know why it's special, you know why you bought it in the first place, you know if it's had any significant maintenance (such as belts/clutch etc) recently - all of these are plus points, so don't be afraid to let them know.
People will probably try to haggle and if you're happy to accept a bit less than your asking price, decide on your bottom line beforehand and be assertive with it - simply dismiss any stupidly low offers and let them see you're not there to be a walkover.
Try and give them as much time as they want to look it over - maybe even point out that you haven't warmed the engine up prior to their arrival (so they can see it starts first time from cold), encourage them to look under the oil cap for mayonnaise, to check the inner side of the tyre for uneven wear, etc etc etc. This shows you are confident they won't find any faults.
It can be useful to see their driving licence and insurance before they take the car out - any time I've looked at a possible car purchase from a private seller, I've offered to bring them with me when arranging the test-drive, just to show goodwill. If you think that's necessary, best to mention it on the phone beforehand, to save any trouble. If this has been agreed and then the prospective purchaser turns up without them - or if for any other reason you "smell a rat" - you can politely but firmly explain that without a security deposit you can't let them drive the car themselves, but will happily take them out for a spin as a passenger and do any accelerating/braking demonstration where it's safe to do so while they sit in the passenger seat. It's still your car till the money is handed over and the documents have changed hands!
Hope it all goes well & you get what you're looking for it.
