Before you set your heart on selling and buying, make sure that you can get what you want for the Z. I have a feeling that $6k will be a stretch....I just don't think that the market is that healthy. Not saying that it is impossible, but keep that in mind. Thinking what you may get and getting what you think are 2 completely different things.....just don't base your decision on "pie in the sky", that's all I'm saying. The car market, esp. the niche market for converted Z cars, can be very dicey. If you can find a buyer at your price, then start thinking about a firm decision, until then you are just farting against thunder. Best of luck....sorry that you are throwing in the towel.....
Also, find out what the cost of ownership of the new car will be:
--maintenance
--fuel
--insurance
--taxes
--Registration
etc etc
and compare that to your Z and what you will need to sink into it to make it what you want. Go about this logically....take the emotion out of it and you may find the decision is easier than you thought, whichever way it goes.
Well...that's my 2c worth.