The BEST DIY diagnostic reader is the Durametric system, which is Porsche-specific, and not only covers the engine control unit, but also most other system like alarm, airbags, aircon, ABS/PSM, instrument cluster, etc, etc. But it's the most expensive. www.durametric.com. Other than that, for your C4, if you only want to read engine codes you could pick up almost any OBD2 hand scanner/code reader from somewhere like fleabay. Bear in mind that some cheap code readers do not show pending codes, only the ones which are triggering a CEL, and I have no idea which one to recommend. Therefore, if you go for a cheapie handheld device, get a scanner, so that you can read the actual values of things like MAF output and O2 sensor outputs. There are also several dozen PC-based scanners on eBay, which aren't too expensive. If you wanted to try a Durametric before you bought one, I have one of those too, and you would be welcome to borrow it for a couple of days. You can download the software from the Durametric site, and then just plug in the cable.
There are 3 different MAFs for 996s. The one for your car is part number 996.606.124.00. The list price of these from Porsche is - sitting down? - £491.33 inc vat. Try Forest Fine or Euro Car Parts as they are much cheaper there.
It's a very easy DIY. You will see the MAF on the top right hand side of the air box, next to where the hose goes off to the throttle body. There are two T20 security Torx screws holding it in. The plug clip is a bit difficult to see on a C4, so I find it easier to undo the screws first, lift out the MAF, and then remove the clip. The security Torx screws have a small pin in the middle, so you need a driver which has a hole up the centre. They are fairly easy to obtain - I bought a set of bits and driver handle for a few quid at a local motor factors.