Another thing to consider...if you are a mechanic - great...........if not, a water pump, (getting to it) costs a mere $800.00 (on a 928.....I think the water pump, new listed at $600.00. So since it falls under the heading PORSCHE......
First of all a 914 has no water pump it is air cooled. The 914 was a ******* child of Porsche/VW and while it has unique styling (love/hate relationship territory) it has a number of drawbacks. First and formost is serviceability. Two primary 914 models were made one was a flat four the other a flat six cylinder. As the chassis design was formed around the four the six suffers from accessability and "balance". Parts on any Porsche as the folks above stated are going to be expensive and the used part/car pool for these models is small. The mid engine concept is good for handling and these cars do handle well (as do most Porsches) when set up correctly. HOWEVER - Z's are easy to work on, parts (new and used) are abundant and depending on what you do to one (even in stock trim)will easily outperform the 914 in all performance criteria. A tricked out Z is more comparable to a 911 than a 914 performance wise. Want to get the idea? See how many 914 internet sites there are.
A 914 is a great handling car, but consider that among most Porsche fans it does not have great respect.
As mentioned above, it is actually a Volkswagen, and was badged as such in Euope.
The 914 has the same engine as my Volkswagen bus..very reliable, but not nearly as classic a powerplant as our beloved Z mills.
Then again, it really is not as an expensive prospect to modify, as the Porsche name would suggest.
How serious are you about autocrossing? Do you intend to prepare a car for autocross only and use a trailer to get it to and from the site, i.e. not street legal anymore? Or do you plan to use the car as a daily driver as well?
If you plan to use the car as a daily driver, I recommend the Z-car. If you are REALLY serious about autocrossing then plan to spend a LOT of money on either car ($10,000 or more in modifications). On a national level, neither car does well against newer designs in stock class.
If you can get a really good deal and you work on all your own stuff, it might be worth it. The cost of the engine work (assuming the 4 cyl VW type 2) wouldn't be too bad, but the suspension and body parts are gonna be alot more expensive than a Z. Even though it has it's own cams and carb setup, the 4 cyl model is a half-breed at best. I had the same choice. I took the Z.
I autoX with the Porsche club and there are some FAST 914s. As a matter of fact, modified versions of Porsches seems to be either 911 or 914, mostly 914s. My impression is that Porsche people choose the 914 for the highly modified varieties. Maybe the others are just too cost prohibitive, I've heard the $2000 clutch horror stories and don't blame anyone if they put their Porsche in a glass jar..."Look but don't touch!"
Boiled down.....I've beat some 914s and been beat by some. My Z is a almost stock 82 280 ZX n/a, only minor suspension mods, motor is all stock except for ignition. But on a National level of SCCA AutoX you WILL be beat up like a tied up dog in either car if you run in stock class. I know Russel has a 83 ZXT that has done pretty well at the divisional level though.
In my humble opinion, in a modified state, pound for pound, dollar for dollar....the Z wins.
O k the Porscha is harder to fix and butt ugly. Ok the Zs looks nice and easyer to fix. One has a motor in the rear and one has the motor in the front. Here a Q for u if u get hit head on which car that you might live. Now try that on the rear hehehe.