Generally, you want to stay with one manufacturer when you want a surround that will mate up with a particular tub for a surround. With any tub, especially fiberglass, steel, and acrylic, you need the thing properly bedded into something solid (stuctolite, mortar, gypsum can all be used) so that there is no flex in the bottom and they are set perfectly level. If that is done, the weight of the person using it is mostly irrelevant. Otherwise, the bottom will flex, and stress cracks are likely to form with eventually the thing cracking (often at the drain). What you have is a standard 60" tub. They are measured from stud-to-stud, not between the finished walls. Now, some of them are slightly less than that, and may need shims if the space was made at 60". Sorry, I don't have enough experience with various brands to make a recommendation. You will need to read the specifications carefully to determine how and if a safety grab bar(s) can be added. Most of them that have a bar in there are NOT rated as a safety grab bar, but it is intended to be used as a rod for a washcloth, or maybe a towel. To put a safety grab bar on, you'd need to first determine where you wanted one, install some solid blocking in the walls and probably add more so it was sticking out beyond the studs to account for the fairly common bowing of the surround (most only meet the studs at the top and bottom, and bow out from there - the blocking would need to be right beneath the wall so when you screwed it on, it didn't bend the wall back to the studs). As to the toilet floor, it sounds like the toilet was leaking for a long time, which rotted the floor. When you set a toilet, if it rocks, it must be shimmed rock solid, or that rocking will mush the wax seal and leave gaps.