dontpressthat
If you are to buy your own rating then all I can say is only do it if you can afford for it to go horribly wrong, I can put you in touch with 5 people who I know personally who it has gone wrong for, and gone wrong in a bad way, it won't buy you a job, but you already know this.
Just ask yourself whether you can afford to lose the money invested, if you can then go for your life, if not then please reconsider, only you can answer that.
Back to the rating, if it was I doing a rating, I would do a 320 rating, why? only a few minor reasons, a) personal choice, b) lots of operators in the UK and c) I wouldn't work for Ryanair if they were the last airline on this planet, so that rules out a 737NG rating
Good luck in whatever you decide
D777