it depends on what type of pball you will be playing! if your looking to get into tourney then i would get an electro marker... the trouble with mechanical ones is that once you get better, they will tend to slow you down... the electro markers are great, so great in fact that many companies do electro upgrades for their mech guns!
if you wanna get into rec-ball... thats woods ball and walk-ons... get a slightly cheaper 2nd hand electro marker..
its a shame to say it... but a Smart Parts Ion is always a good starter... they are relatively cheap, run on air and CO2 and have tons of aftermarket accessories and upgrades! for the Ion try this site out... quality guys:
http://www.lpspaintball.biz/acatalog/SMART_PARTS.html
another thing... i would try and get an AIR tank not a CO2 tank as many sites will only take AIR (compressed air)... a new air tank is expensive at first but it saves you in the long run.. trust me! try a new one from lips... Ions can run on High Pressure or Low Pressure...:
http://www.lpspaintball.biz/acatalog/Systems.html
that said the 2nd hand market, ie, the classifieds on these forums is a complete buyers market.. you can pick up a good 2nd hand Ion cheap... or get a more advanced one if you like it and wanna learn it!
THE BEST THING YOU CAN DO IS TRY PEOPLES MARKERS... peeps are helpful enough, so just ask them if you can hold it and fire it to see how you like it...
hope this helps!
Scott B