There are a lot of loft units in Lowell that have high ceiling post and beam big windowed construction, but not all of them are in old 'mill buildings'. For example, the lofts in downtown around Merrimack, Middle, and the north side of Market street were originally office buildings. Canal Place on the south side of Market were originally mills. Some buildings are more centrally located and walkable than the larger mill buildings near the river. Some buildings have far fewer units than the mill buildings and have sidewalk level shops, restaurants, etc.. Some of the larger mill buildings are very isolated from sidewalk life. Check out the block where Athenian Corner Restaurant is located for a taste of 'sidewalk life' in Lowell (and visit the National Park orientation center across the street). The character of downtown Lowell varies from block to block, day of the week, and hour of the day, so pick your building carefully.

The occupants of the buildings depend on many factors, and there is a big range of demographics. You will find what you're looking for in some buildings, but not in others. There are people like you who're very happy living in downtown Lowell. Some blocks have near perfect 'walkability scores'. This is one of the principal advantages. For example https://www.walkscore.com/score/207-market-st-lowell-ma-01852 The canal and river walks let me walk, get away from cars, and not have to cross many streets. It's really a walker's paradise. I rarely use my car anymore.

Google maps is your friend. Pick a location and do a search for like 'restaurants near here'.
https://www.millcityproperties.com/ specialize in rentals. Some buildings have a mix of rental and condo units. The developers gradually sell off a few rental units as condos. Some buildings are all condos. This allows you to 'try before you buy'. There are also condos available for rent that are owned by individuals.
Downtown Lowell is economically and ethnically diverse down to the block level. Whether that is a pro or con is a matter of individual preference. I think it is a big plus.