The S30 cap is for a Sodastream gas bottle which, to be honest would be cheaper than gas bulbs in the long run, but means you'd need a regulator as well.
With regard to your question about serving temperature yes, you could take it out of the fridge when you want to serve it, but unless you're very, very careful and/or lucky, you'll disturb all the sedinent that's settled out.
Therefore, you'd need to let it sit at least overnight to let it all settle out again, so you'd need to find some way of keeping it cold, unless you can find a way of pouring while it's in the fridge.
Over the years I've not read as many positive things about pressure barrels as I have about Cornelious kegs etc and only used the one I had a couple of times for ales with probably not much success. That's not to say they're not great and there are threads on various forums where users have great success.
As you want to serve a lager, personally, I'd buy some bottles of beer (the thicker the better), some caps and a capper and if you've not got enough empty bottles by the time you need to bottle the beer, put the rest in sparkling water bottles. These will tolerate higher pressure and be easier to keep cold.
Save the pressure barrel for ale or see if you can return it.