Welcome...
I have a '95' B1700 (4x4 gear drive) so I can't speak for the HST model but it is a great little machine...I'm not aware of any chronic issues...
There is an issue with some configurations of finding replacement tires for the front...
The biggest issue I have is the lack of overall weight...I was originally looking for a tractor with a bit more HP (20-25) but after putting over 200 hours (typical gravel road maintenance, tilling etc.) on the B1700 the only thing I have found lacking is weight/traction...so I have learned take things a little slower and develop techniques to overcome the weight/traction issue...
You can find complete parts list. etc online (start with Messicks)
Egon, You are correct also I thought B7100 with the axle flanges too
I will say this tho, I have the 2001 B21 - similar vintage to the B1700 just heavier with the frame and BH and so far has been great.
The only issue I have seen/heard with these B's is a push in plug that can leak behind the bell housing above the HST driveshaft.
Its really hard to see - my B21 has it and is not accessable but the B1700-2100s have the same press in plug that will eventually leak - it appears at first glance the shaft seal to the HST as this plug is just above that shaft.
So if you have a slow center leak of hyd fluid thats usually the issue. On the B1700-2100 these plgs can be replaced without splitting the tractor.
The B7100 is a solid, simple tractor. The hubs could be better, but you can either replace them with an improved design (New Kubota Rear Hub Assembly B2410 B4200 B5100 B5200 B6100 B6200 B7100 B7200 ++ | eBay), or maintain yours by checking to see that they are tight.
Probably the only detraction, at least in my opinion, is the lack of position control on the 3 point hitch. It is either up or down, you have to jockey the lever to get it where you want, instead of it moving progressively as you move the lever. I am used to it on mine, and I wouldn't get rid of my tractor just because of that.
I also don't like the turnbuckle stabilizers, but the B7100 is hardly the only tractor with those- most new, small tractors still come with them. I make replacement telescoping stabilizers, so I always have a prototype to try out on mine.