ELM.
I had BI-TKR on 11-13-08 like you I am about 100# overweight. The surgery went fine and I started PT day one just stand,day 2 stand and walk 30 feet with walker, day 3 walk another 100 feet. After that I was transfered to a hospital rehab facility for 7 days. when I was released I could walk with a walker, navigate ramps and low stairs, get in and out of bed, in and out of chairs and other daily activities. The PT was no cake walk but it was easier than I expected. I am now 14 weeks post op and have lost about 30#. I had the same reservation that you are having but the DR. was very upbeat and assured me every thing would be OK. I don't have any experience with nickel allergies but I know my knee is a Smith and Nephew OXINIUM which has a highbrid metal and contains no nickel. I am sure your DR. has taken that into account and can provide you with info on the exact implant he uses.
Some things you will need at home are a raised toilet seat, a shower seat (adjustable), a walker, cane, ice pack and heating pads, a piece of 4 inch thick foam rubber(place in chairs, recliners, any seat that is low and hard to get up and out of, I carried mine with me everywhere for a couple of weeks).
I dont think there are many people faced with major surgery that don't have second thoughts. I was in the pre-op are already preped and ready, laying there waiting to be taken to surgery and thinking I can still change and have just one knee done the next thing I new I woke up in recovery with two new knees.
This surgery will change your life, I am doing things now that I have'nt done in 10-15 years. Each day I discover new things that I can do without pain.
Max
GOOD LUCK! Trust you DR.