I was 33 when I had my daughter & almost 40 when I had my son & I admit it was hard going back to dealing with nappies etc etc & as I too have serious back, hip & knee problems among other things I know what you mean re worries about coping with the pregnancy & birth much less the carrying (esp.With all that baby gear!!) the baby afterwards - & they do insist on growing & gaining weight before they can walk etc etc!!
It was hard, to go back to having a baby around & it has taken a toll on my health but I wouldn't be without him for anything, all the pain relief or lack of health problems in the universe wouldn't make up for not having him.
In terms of surviving the pregnancy, take calcium, iron, multivitamin & damn there's another one that you should be taking before you fall that reduces the chances of health problems for the baby, damn it. Get rest but also keep up gentle exercise so as to stay fit, eat healthy foods, prepare as much as possible before hand (like getting nappies etc etc etc so that you don't have to worry about all that for abit after the birth).
For both of mine I chose to bottle feed & in terms of age & health that was a god send the second time around as it meant my husband could do more & that allowed me to get rest when I needed it. You also might get your older son's help (but don't count on it, at 14 he has his own life & friends & you don't want to risk him going off the rails due to jealousy & annoyance at always having to babysit etc).
As to improving your chances of having a girl>> sperm that create boys swim fast but die sooner, sperm which create girls swim slower & conserve their energy so last longer therefore if you have sex right at the time of ovulation you're more likely to have a boy (which is why the vast majority of ivf babies are boys) & if you have sex a couple of days before you ovulate you increase your chances of having a girl. But nothing is foolproof. I've read other ways to improve your chances but don't recall them maybe trya search engine & something like how do I increase my chances of conceiving a girl.