I'd be really sketch about buying ketamine online. Agreed, please test a small dose before going any further.
Be sure you know what you're really doing here.. k holes can be very confusing places and you will be fully immersed in it for some time and unable to get out. I know that I threw up my first time k holing, but everyone is different. And no, please don't do 225mg at once, that is completely unecessary and, seeing as you are a first timer, will send you completely over the edge. When I first started sniffing k, it only took around 100-150mg (everyone is different) to put me in a hole.
Just going to put this out there right now that there are no "what should I/did I take" threads. No one here can tell you what exactly to take, since, like I have said, everyone is different and different people react in different ways to different substances and dosages. Yeah. That's a lot of different, isn't it!
You asked if you should take a 50mg bump first. This sounds like a fine dosage to start off with. Wait perhaps 15 or so minutes until you know exactly how you feel, then try taking another bump of the same size. Wait again and see how you feel. Continue doing this until you feel you have reached the effects you were searching for.
Don't go and do mass amounts of ANY drug your first try. It's silly and quite irresponsible. You could harm yourself, because you are unprepared and have never experienced any of K's effects before.