I've been down the 11-48 road a couple of times.
They are one hell of a good shotgun. Many of the parts are somewhat common with the 870.
But there are some annoying issues with trying to tacticool them.
The threads for the mag cap are the same as 870-1100, but the threads are longer, so most extensions thread on but will not bottom. They need to bottom in order to hold the barrel on properly. I even got my gunsmith to do some machining to the extension and it still wasn't right. It had float/slop despite his best efforts. I did use a big lockwasher flattened out, but then you lose the effect of the little serrations and ball to retain the cap or entension...so you have to keep tightening it.
Yes, you can only add a plus 1 extension due to mag cap laws- 5 rds max in the tube, 2 3/4". It made me wish I had a 3.5" gun like a VersaMax that will run reliably with 2 3/4" shells.
There are no aftermarket stocks or forends for the 11-48.
The forend is special because it has a metal liner because of the big recoil spring. 1100 or 870 forends do not fit.
The buttstock is different than 870 and 1100. The tube with the spring in it is a different length than an 1100. You will not find this information anywhere at all on the internet, except right here in this sentence.
So, if you don't have a good buttstock and forend, or they're cracked, it's hard to improve the situation. Not impossible but hard. You are almost stuck with the stock butt and forend.
I used an S&J Hardware 1100 extended cocking handle, which worked great. The detent method is different and you might need to make a detent notch on the new handle.
If only...the mag tube threads were shorter and the buttstock spring tube was the same length as an 1100. Because the 870/1100 buttstocks slide right on and fit up nicely....but there is no way to hold them on.
It depends on how much buggering around you want to do with it.
Also, I enjoy 12 gauge recoil and riding it...but not everyone does. Now the 1100 gas gun soaks some of it up with the gas system. The 11-48 gives you the whole recoil impulse, sproing.
Really, buying a good, little used 1100 makes more sense...because you can buy off the shelf items to dress it up.