Since you're a new player I wouldn't recommend a gas gun, they need a high level of maintenance, a fair bit of knowhow and are very expensive to run, magazines are very expensive and often hard to get hold of, you have to think about gas as well as ammunition and they don't work well in cold weather.
Not to mention the mags more often than not hold less than 40 shots.
On the topic of getting magazines for it, I don't think I've ever seen any mags anywhere that would work in that, I've never even seen one of those gas MP5Ks at all, to be honest.
Which means all the parts you need for it will be rare as all hell.
If you want an MP5K you'd be better off getting an electric one. You'd be better off getting an electric version of anything actually.
The first gun I ever bought was a gas M16 and it was the worst gun I've ever had, I sank about a grand into it before I gave up and sold it on. They're just not worth having really.
Just my two pence.