I voted for the 42 based on parts availability, but I'd stock up on spare parts for which ever you choose. None of these guns were meant for a lifetime of shooting without piles of spare parts.
My 34 is really a reliable gun. I numbered all my bolts and barrels. After a couple of long range sessions, was able to put together the most reliable combinations.
All combinations would function, but some with constant failures on sustained bursts.
All in all, if one is not going to put some real backup money into a 34, might be best to choose something else.
My 42 runs smoothly with Romanian, Portuguese, or the 70s Yugo. For the harder primed 50s Yugo, I SLIGHTLY sharpen firing pins to get a deeper indentation on the primer. Still not 100% reliable, but its the cheapest!
Always run a 42 with the bolt safety or positive locking spring if you are shooting surplus to control hangfires.
The HK is like the 42. A vicious gun to shoot if not using a tripod. A belt now and then from the hip, shoulder, or prone is fun ( it gets old after a while), but for actually hitting something (And actually see what you are hitting. Try either one sometime while shooting prone and tell me what you see. Only then will you know what I mean) , a tripod is the way to go.
Don't know enough about spare gear for the SAECO, but my experience around them has always been positive and they are definitely the most user friendly for shooting enjoyment.
With the ability of Congressional/Executive actions and/or laws to put a halt to further transfers at any time, I would not consider any NFA to be an investment at all.
Life is short. Just get the one you like if you really want it!
My take, JT