I agree with Harry, keep the MF135. But no reason why not to add a newer machine to the mix. The newer ones really are better and more comfortable. But won't match that 135 in reliability.
If you look newer, look in the 2000 to 2012 era. The last era before emissions control went onto everything.
My good friend had a Massey Ferguson 135 he made hay with on 8 irrigated acres. It was bought used from a mowing operation about 1990 and was a lovely machine - a gas continental - with the standard transmission and no optional power steering. It had about 5000 hours and ran like a clock. That 135 never needed anything except a fuel pump and periodic carburettor cleanings for almost 30 years. Nothing elver broke. Making hay on a small farm is the perfect job for it. Unfortunately it was in a barn that burnt and totally destroyed. Otherwise I am sure that it would still be going strong.
Not having power steering, it was never suitable for a FEL, and that was the only drawback - but it was the perfect 3pt & PTO machine. He used a rear 3pt bucket when carrying things. The main drawback was that as he got older he really needed a FEL to save his back - but that Massy was so nice he stubbornly refused to change. It has become a family member....
They are an unusually heavily built 45 hp tractor. HOWEVER, IMHO, you will need to go to about a 60 hp Kubota or similar make to have the same combination of stability, low rpm torque, and quiet power. Kubota makes a number of 60 hp tractors, that size is one of their best.
I'd keep the MF135. You won't get much for it today, but even today nobody makes a better 3pt open utility tractor for 3pt work. And yes, do get a modern tractor. Get power steering and modern conveniences.That's what we did. It's much more comfortable to operate.
We kept the old one for a few chores and got a modern Kubota too. 15 years ago, and very glad we did.
rScotty