I just came back from Manali, had, like quite some other bikers in these countries here, an accident, someone crossed the street with his scooter right in front of me without looking left or right, so no chance of avoiding it, nothing serious happened to me except a double-sized right hand and hits here and there, but the bike looks pretty bad and will be shipped home now. So drive SLOW and be EXTREMELY careful, as I thought I was, but still not enough. Thats by the way, the road to Manali is beautiful, the road from there to Leh supposed to be far better even (it is still closed now, the biggest disappiontment on my whole trip so far), but requires good timing, as short after the opening of the passes the rain- season starts, possibly damaging the road again because of rocks coming down, so for a really enjoyable ride you should somehow catch this 1 or 2 weeks. Same for the way back.
Renting as well as buying Enfields is possible there, lot of shops doing this, but I know nothing about prices and conditions.
If you fit two people on a bike, waht is possible without problems, depends on your luggage, I would recommend bringing as little as possible, really cheap guesthouses everywhere, no need for cxamping excepty you want to, and Enfield- sares available everywhere, but concerning this better speak with the guy renting/selling you the bike. What u need is a second pair of clothes, a toothbrush, some shampoo and washing powder, the rest is luxury. And, maybe some biking clothes, my helmet, and special jacket and pants saved me a lot in my accident. Think about it, even if it is unconvenient sometimes and everybody else gives a shit.
Thats all I can advise, have fun up there, such a pity I couldnt go!!