Is this a good first car? I'm happy to learn and work on it but will it be costly to keep running?
This depends greatly on the condition of the car you buy. This does not apply only to MX-3's but to any used car on the market. A car that has been properly maintained will have a lifespan 5 times greater than a car that has been abused and neglected. As with any older & used vehicle.. it will need maintenance.. and the prices of these cars generally reflect not only the mileage and condition.. but often include maintenance records.
There is truth in the old saying "You get what you pay for". Does that mean you cant find a good car cheap? No.. you can.. but it requires patience and the knowledge to know what is a good deal and what isnt.
Why are they generally cheaping then other vehicles (civics, 323's of same date)
I havent noticed that the prices have been getting less in my area.. so I think this one is going to be dependant upon where you live and the demand for the car in your area.
and lastly...
Are you happy with the power given by the stock engine/parts? (what is it comparable to?)
That completely depends on what your intention towards the car is. If you are going to use it as a daily driver and dont have any grand plans to race it or autocross it.. then the cars seem to have average pep and power for their size & class of vehicle. Others may say they were under powered from the factory.. but I feel this was offset by the superior handling performace the car came with that outclassed anything in its class at the time of its release.
Now if you are wanting a Ferrari for $2500.. it aint gonna happen
For the most part.. the questions you asked can really only be answered either by you or by the car itself. I can say with some confidence that if you buy one.. you will love the car

And as for the value? Well this website and the people on it make the car just that much more valuable.. sure there are other forums on the net for other cars.. but none are more like a family than this one in my opinion. When I bought my first MX-3 a year ago.. I bought it because it was in decent condition and with a bit of maintenance.. I knew it would be reliable for a good long while. Then I found this website.. and suddenly.. my little commuter car pretty much became an obsession LOL Now.. a year later.. I have bought a second one and this one will be my toy sports car
Research is the key here.. these cars are pretty easy to work on in general.. if you are willing to read and learn.. what you can do with your car is only limited by your imagination. Cheers
Tunes67