I can add some backup to the SAR info - I am from a FW ALM background and I was offered a slot on a SAR pre-select at SARTU. There are also a number of ex Air Engineers on the SAR force so you definitely don't have to start out as a RW Crewman, although I am sure their background helps a lot. i know it could be a long way down the line but I believe Shawbury are very helpful to people wanting to cross over to SAR and can offer something of a rotary famil to help people before going to Valley for selection.
As regards your age, don't worry about the training. "We are all individuals."
You will either find it really hard, really easy or as most people do, fairly challenging. Who knows till you try? Your maturity will definitely help though.
As regards the GF, it's really up to you but are you possibly willing to lose her to chase a dream? Only you can answer that, but you can always opt to leave, I believe up to OCF/OCU, and the time spent away during training will give you both a good idea whether you are happy spending time apart.