As Cannon Fodder has said, it depends on what you are looking to do in the sport, whether it's standard gaming in the woods/scenario based games or if you are looking for more tournament based activities.
With Scenario based games, after 15 years of playing them, I have found the easiest marker to use and maintain are the Tippmanns and the BT models. Very straight forward and very adaptable. You can pretty much alter the whole look to how you want and there are plenty of options out there to assist you when it comes to these markers. Prices of these, new, vary between £100 to £350 but also like Cannon said, it might be worth buying second hand at this stage and then determining what you really want from your marker.
For the tournament side of gaming, I've only just got into it myself so I will leave the advice to more veteraned gamers around here but I know the team I hang around with use EGO's and Eteks. Slightly more pricey but hell of a marker. For myself having only just got into this side of the PB gaming, I've headed in the direction of Proto and am using a Proto Rail 2011 although this is a get you buy marker for something a little more substantial down the line. There are a lot of nice markers out there and I'm sure the people around here will be very happy to assist you in making your decision