Moving up the ladder and getting your first jet ride is a hit and miss affair that falls largely down to being in the right place at the right time, and it helps if you know the right bloke!
Whether one should pay for a rating or not is a perennially asked question. My guess is that most would advise you not to do so, and probably for good reason. Unless you have some kind of guaranteed job after having paid for the rating, and have covered the line training (if required as per EASA), I would agree with the argument that it is not a good idea.
There was a time when the CE500 rating would be a good one to get, and if one funded their own training it used to be a realistic proposition that work would come your way. That was pre EASA, and in the meanwhile many of the older Citation models have dropped off the register. Picking a type to self fund, if you wanted to take a punt, is now not so easy...there are more types flying, and it is likely that you will end up with a rating that is not so desirable.
My advise, for what it may be worth, is to invest time rather than funds. Spend as much time as you can meeting and networking. Pop into offices, ask to say hello to as many employers as you can, and keep showing up as often as you can. Most CP's I know will give you credit for trying, and I certainly do when it comes to hiring. I'm sure also that they remember the hurdles they had to overcome when they were looking for their first jet ride, and going out of your way, rather than simply replying to ads, will appeal to them.
If it helps, I probably was one of the most undeserving co pilots in terms of time and experience when my chance came along, but having met the guy soooo many times I'm sure he would have felt guilty if he did not give me a chance. Having got it I made sure NEVER to let him down, or any of the other employers that followed. After being given a chance,that is key to keeping the dream alive!
Good luck.