Welcome!
I think you've chosen a GREAT breed! I'm going to assume your fiance will be a huge help in the 'lifting' department, so if there are any emergencies, you'll be covered. Also, if you have some good neighbors, that might be good to have as a backup, too. I might not have a constant disability - but with a 165lb dog... there's no way I can lift her by myself. I'd have to design a sling of some sort to drag her into the car if she became ill and I was here alone... or...I'd grab a neighbor. We have lots of dog-friendly neighbors around that I know would help me.
We were looking for a big couch potato, and now that she's over 3 years old (almost 4), she finally fits that expectation. I was amazed at her energy levels as a puppy, though... even through her second year, she was gung-ho for long walks (2+ miles) at a jogger's pace (for me, trot for her). She still likes the long walks, but if we don't go, she's content to nap the day away.
For a few days, anyway... then she starts to get a bit depressed and pitiful... not pushy anymore, but the guilt-trip is also effective at getting me up & moving. HA!
We did a few puppy classes and a puppy playgroup for her early socialization, and then went on for her Canine Good Citizen certificate, which was a first for me, and very rewarding! We did all-positive clicker-based training, which she excelled at, and was very fun for me, too.
Now her "socialization" includes tailgates at college football games (LOTS of loud people around), camping trips with friends, and treks to a dog park (maybe 3-4 times a year... or less, we have a canine flu and some rat-poison issues going around here lately). But, her regular social time is mostly just games with her two adults in the house & yard, and sniffing the bushes in the neighborhood while out walking.
I agree with Smokeycat, though, socialization is more about instilling confidence in the pup in lots of different situations with lots of different objects, so they don't FLEE (or attack) the unknown. It's not really about getting out with other dogs for tea-time. Many mastiffs are very happy to be loners in the dog world - as long as they have their people nearby. They do NEED their people though! You can "socialize" at home by introducing new stuff every day (umbrellas, loud noises, new toys, etc.)... and then take one day a week to get out and explore new places, situations (crowds, pet store, parks, water, forest, etc.) and walking surfaces, etc.
I'll just put in a plug for your local shelter dogs... most of the dogs surrendered to shelters are there due to landlord/housing/moving issues, and not behavior problems. I follow a local group that focuses on senior dogs... at least half of them are looking for a new home because their long-time human has entered a retirement home that doesn't allow pets, or they've passed away. I'm not sure a senior pet would be the best match for you - they often come with their own set of age-related disabilities... but just throwing that out there... if you think a puppy would be too much on your health and your home, there are many very healthy, well-adjusted dogs in shelters or foster care waiting to be rescued.
Good luck in your search - and let us know what you end up with - hopefully we'll be seeing you around here soon with LOTS of Puppy Pictures!