Not really sure how relevant SMOH is on a rental. I would only worry about that if I was the one responsible for replacing the engine. Two thoughts come to mind about flight instructors that might help you decide if you really need to ask this one.
Number one, anything that happens to you while flying the airplane is going to happen to the instructor (excluding solo time) and he or she will be flying with you a lot. The instructor is not likely going to get in an unsafe airplane if he knows it.
Number two, most Instructors seem to really be worried about liability and responsibility. Had one that use to remind me of the risk he was taking by instructing all the time. If you ever do this or if this ever happens down the road you know they are going to come back and look at me…. Being that the CFI is also the airplane owner he is taking on a decent amount of responsibility in both instructing and renting you the airplane. I think he will make sure the airplane is in the best possible shape.
I have rented airplanes in the past that I have known total times on were extremely high. Learned later a couple of them were well past TBO. All airplanes have quirks and problems every now and then. The worst I have encountered is an alternator failure in the pattern on a rental. The thing you need to look for though is how well the airplane is maintained. If problems are noticed how quickly are they fixed? Is there a way for you to report problems (squawk list) with airplane and is doing so encouraged? Are there things on the airplane that don’t work? Flew on once with a broken primer knob, and again several years later and it was still broke. Caps were missing on the tire stems too, so I pointed it out and got a, so….
Most rentals used for instruction are going to be under 100 hour inspections. So they are going to see a mechanic quite often if they are used a lot. It doesn’t bother me to fly an airplane I know is a little over SMOH as long as I know it is getting good maintenance. If you notice something wrong on the airplane by all means point it out. Curious about the type of engine, how often the oil is changed, if that wire is suppose to be hanging there, or what that duct tape is covering by all means ask. Something you want to learn about the airplane don’t be shy about it.
However, I think I would keep the SMOH question to myself for now. That could be a turn off or might make the CFI think you don’t trust him (without good reason). You might ask it later once you have made friends with him, and at that point he might be glad you asked the question. If you don’t trust him for any reason, don’t fly with him, no matter how good the deal is, even if the engine was overhauled last night. I once flew with a CFI I had been told had a drinking problem among other things, because the airplane rental was much cheaper than anywhere else. Don’t think he was ever drunk around me (and don't know if the rumors were true), but he was a terrible CFI and I just wasted my time and money there. The itch to fly can make a person do some pretty crazy things, just keep your head on. So, curious, why the SMOH question anyway…..?