The problem with aftermarket floors is they aren't an exact fit. When you buy replacement floors they have to be shaped and trimmed to meet up with the transmission tunnel; if you look at the shape of that section you'll see what a pain in the ass that can be. Best angle of attack is to cut the original floors back to somewhere before they start curving upwards (if they aren't that far gone). It's still a ton of work though and will cost a fair bit if you are paying somone else to do them.
The other problem is where the floors join up to the rocker panels; if the rocker panels are in bad shape internally then you might find the welder blows straight through them when you go to put the new floors in. If that happens then you suddenly have a much bigger issue altogether, and if you are paying someone for the work then the cost will skyrocket.
Not trying to discourage you, but it's something to be wary of.
And to demonstrate...
...this is what happens when you weld patches over rust. Basically I had to remove 3 outer rocker panels on this side; clearly when the original one rusted through somebody welded another one right over the top, then another over that when it rusted through again. And the rocker panels looked spotless from the inside of the car. In fact the area the floor was attached to is paper thin and disintegrates as soon as you look at it!
I'm about halfway through reconstructing this section from scratch, it's taken a week so far...