No, the floor joist are not adequate. The rule of thumb for residential floor systems is the span of the joist does not exceed the height of the joist. So, a 2x12 maximum span is 12', 2x10 is 10' and so on. Plus, you mentioned added sub floor and ceramic tile - this just adds to the live load on this floor. The supporting wall for the 2 levels above is most certainly going to need the floor joist under it supported. You will have 1 floor joist carrying this added weight of the new wall even if it does not carry much weight from the above 2 floors already in place. Then you add the occupancy load and tables/chairs for a restaurant. Your floor is going to shake every time someone walks across it. What you need to do is go into the basement/or crawls space and install a floor beam perpendicular to the existing floor joist and install it right at the center of the span. Anyway - No, it won't hold the load you have in mind.
This is something that needs to be visually inspected. What it comes down to is, you would need permits to make this conversion and so the county is going to impose requirements. If you are serious about this project you should consider finding out what those requirements will be. If they include a structural engineer's drawing and approval, then you might as well get that ball rolling. I don't think you can go on anything anyone can tell you here. Sure it sounds like it has the potential of being adequate, but there is also potential for additional support needing to be added below, such as a beam perpendicular across the center on posts. Consider the combined weight of occupancy and service equipment on the entire floor. Look for any signs of stress, rot, infestation, water damage, trades over drilling, sagging, splitting, etc. These will be the determining factors. Chances are good that you have adequate support but my opinion on the matter is useless to you.