My donor is a 1999 2.8, Fantastic drive. The 3.0 would probably be better but usually much more expensive.
Mine has A/C & as John James has done I built a box to hold a short chassis radio, looks quite good although I think John's looks better. Saying that, mine has a lid under which I have my heated seat switches & my A/C switch (a bit tight, but it works).
Steve's thoughts on the alloy windscreen surround is sound, but cheap enough to buy from BMW (£100), that is what I have done.
I also have cruise control, which I use all the time as most of the shows we attend are motorway bound, just a small modification to the steering column surround.
My donor colour was dark green & 250 is light blue, noticeable when bonnet is up but not glaringly.
I feel that there should be a 6th gear on the Z3, just feels that it would easily take it. I ran mine for a couple of months before starting my build & also changed all suspension, brake discs,etc. & steering joint, easier without all the bodywork around. I had to put the original springs back on all around as mine was grounding over speed humps & even some country roads, all okay now though.
Just take time getting gapping, etc right. good luck mate & look forward to seeing your finished car