I did my medical last month and was worried like you as I had knee reconstruction in past, I don't have any problems since my operation my knee is 100% if not stronger (2 titanium screws there for rest of my life). The only reminder is a scar. The doctor performed few exercises and stretches to make sure its fine I was told everything is fine. I was offered ground school 3 days later. Good luck
in regards to first aid certificate. I didn't have First aid certificate or RSA when attended my AD. When I got my golden call I was offered ground school but was told that I have to obtain RSA and first aid cert before my first day at GS.
ALL the best