I've seen a lot worse floors than that in my time (40 years of aircooled VWs) and as others have said, that looks very much like the result of a leak somewhere else rather than the cause of your problems.
Welding a floor isn't hard, so maybe it's worth you looking at buying a MIG welder and patching that yourself - if it was me, I'd weld the drain holes closed as well (if you are welding from the inside, make sure you use some form of heatshield on any pipes underneath the area you are working)
I'd also take a look at the sills - if you can lack the car using the jacking points they are probably (please note I said 'probably') OK but they are worth inspecting/repairing and sealing. They are not as easy to repair as a floor but as long as there is some decent metal to weld to, plating them isn't that bad.
Buying a welder and learning how to use it can save you a lot of money - my (budget) MIG cost me £200 five years ago - it paid for itself the first day when we used it to get my other half's old W123 Merc through it's MoT
