lizzzi, we may be having some cultural differences? OP was talking about moving into their condo up the stairs at age 50, which I think is still an age for keeping excess weight off, being active and exercising regularly - the average person here can reasonably expect to be active and healthy up to their 70s which would be 20 years worth of up-the-stairs living before possibly moving on again. For the other advantages mentioned by the OP (overall location, and ability to FIRE earlier), planning to move into the condo when aged 50 seems OK to me.
Within my neighbour's bungalow, everything was of course on the level and accessible, including wide doors and a re-jigged bathroom - I agree with you on this being a good idea, if it's possible. Also, there is very little snow or ice here, and the home help aides my neighbour had from her mid/late 90s onwards loved her, loved the views and didn't have any issues coming to her house. Ambulance crews have learnt to deal - it is pretty common around here for there to be access difficulties, given the general terrain and age of many properties.
Outside stairlifts are not uncommon, and often available second hand. The only limitation is that, unlike inside ones, they have to be a straight line only. Inside ones come in a great many configurations - given the age and quirkiness of UK housing stock, they need to.