Someone told me that if I am having piston rings replaced, it is very important to have the cylinders "bored" or machined as the piston rings are "set" in their particular cylinders.. Is this true, is it super important to get this done?

You should probably mic the cylinders really well to check for roundness, taper, etc, and make sure the crosshatching is still good. If not, then some machine work would be a good idea. If it had coolant or water in the cylinders, or other things bouncing around in there, you should machine any damage away if there'* enough metal for that.

How many miles are on it?

98000 km or 61000 miles

That'* not a lot of miles, unless they were all in downtown New York City or the like. It might be okay without any work on the cylinders, but the only way to know for sure is to measure and verify all of it is within tolerance. Before measuring, I'd look to see if the crosshatching in each cylinder looks even, without any irregular smooth or scored spots.

Mechanic says that rods are bent as are some valves. He can replace valve but says that piston rods are hard to put together if I buy new and he needs a tool. What type of tool and how expensive are they? How difficult is attaching pistons to rod if the "tool" can be purchased?

Take it to a machine shop. Your mechanic should know a shop.

You need a machine shop to assemble the piston? I looked at youtube and the piston has a tube put in the head with ring snapped on each side..is this something that has to be done by machine shop? He said that the block was good.

If you look you will find the rods are for a particular cylinder and you can't just throw them anywhere. Do what you want.

Sorry, I don't understand. "Rods are for a particular cylinder", I thought all rods were the same...You mean there are differences between the 4 rods? ...So I have to get a set of 4 rods (each one designed for a particular cylinder) rather than replace only the ones damaged ?

You need to index the NEW rods to the new bearings and crank. The clearance between the side of the rod and the crank is critical.