Listen, one day you might consider putting that 42/32/22 on that bike. Makes a great touring crank when heavy. But need a wider bb and some 8 spd shifters....if the hub can take 8 speed...
The weight depends on the crank. If the crank has steel rings, yes, it’s heavy. But if it uses aluminum rings it can be competitive with a road triple. If the crank is a Hollowtech, it’s even lighter.
As for the bottom bracket, the spindle may need to be longer or it may need to be shorter. Again,
it’s best to measure it. I’ve often had to use
shorter spindles when I use a mountain bike crank on my touring bikes.
I've toured a lot with a mtb triple, and heavily loaded it works very well, especially with hills. I run out of gears at about 50kph but that's ok, pretty small percentage of time we ever go above 35kph.
you could just leave the bike with 7 speeds, no issues at all running 7 speed chains on 8 speed chain rings.
but you would need a wider bb, and shimano makes an inexpensive sealed unit called un24 I think, 20 25 bucks. Just have to get the chain line figured out, hence the wider , longer length axle.
no matter what, if your bike has the original ball bearing bb, if it's not in good shape, replace it with a un 24 or whatever it is. Sealed bb's are so much better than ball bearing ones, install and forget for years and years.
don't know what tools you have, but most is doable at home. Removing a ball bearing bb cup can be a bear though. Had to use my vice and rotate the entire frame of my wife's commuter bike to get the old bb cup out last year when I finally put in a sealed bb, the 20 25 buck one I mentioned.
Another alternative is to go with an external bearing crank. A little more expensive...check Fleabay...but easier to work on and install.
On all of the 64mm BCD mountain cranks, you can convert them to a 20 tooth inner ring. The 20 is too small to bolt straight on but with just a little bit of filing. Here’s a video on how to do it on an external bearing crank
A 20 tooth 64mm BCD is fairly rare but they can be found. I have done this on several bikes and it works like a charm