A DIY can do this him/her self.

Course, if your not a very qualified DIY - then possibly get a few more experienced friends to help you out, while you learn, and have a few pizza parties...

But, I'm of the mind that a DIY can do most anything....

Don't have the slightest idea of what your house looks like, or the structure above the sunroom... the roof situation etc...

If you want this job done QUICK - then you should probably hire it out... when not experienced, DIY can take a little more time. Actually, even when experienced, DIY almost always takes more time...

Go to Home Depot, and purchase "Home Improvement 1-2-3"; and also locate (published by: Creative Homeowner - "House Framing") The Library or a few good "How To" books are your friends. Wealth of info...

The two layers of plywood are probably for strength, and are installed with long edge seams perpendicular.

Replacing the plywood is very simple. Requires a regular skill saw set at proper depth, and a toe kick saw. (A toe kick saw) allows you to cut extremely tight (flush) to a wall. A cheap one is available from Harbor Freight for $60 (#94626)

Anyway - you cut out a larger area of the plywood then damaged, and then replace it. Simple to replace plywood.

More of the question would be the condition of the joists which the plywood is mounted on... Do those joists have rot?? If they do - they should be replaced. That is more work...

Plus, the wall studs. How are they?? Since you state the leak has been going on for some time... it has been coming from somewhere... Probably flowing down thru the wall... So possibly a window is leaking into a wall, and the cripple studs beneath the window are damaged as well??

Like you stated - you must find the source of the leak...

Black & Decker also publishes an excellent roofing book: "Roofing, Siding & Trim" (isbn: 13:978-1-58923-418-5) Great resource for putting down a proper roof, or for fixing your roof... Or for the techniques in weatherproofing the side of your house, window trim, etc...

As you can see - this can get to be an involved job... You might also unfortunately have termites or other pests (carpenter ants) - since they need moisture to eat wood, and like wood with moisture around...

Not trying to scare you. I do most everything myself... after reading...

Your job can be simple, or can be difficult. Just depends on where the actual leak was, the water path it took, and the amount of time that it has been going on...

But - like I stated at the beginning - I believe most people can DIY if they want... just depends on the amount of time they have available, and how much they wish to persevere, and read about what they need to do...

Good Luck. Fixing leaks, and putting in some new wall studs, and weather proofing - are not a hard job.

My wife who used to be involved with RV's a tremendous amount, informs me that one of the problems with aluminum walls - is that a tremendous amount of RV's used to have rotting out floors, because the aluminum framing would 'sweat'. It wasn't insulated, so water would condense upon it, and then drip down to the floor beneath... rotting out the floor....

Possibly what's going on here...

A small section of plywood could be cut out here, without the wall above it - sagging. Yes, the aluminum wall is probably resting directly on top of the subfloor.

If the aluminum wall studs have been 'sweating' (condensation forming on them) - then you might wish to slowly work on small areas of your floor - and put in some sort of moisture barrier between the subfloor and framing; with a periodic outside 'drain'. You would have to read about that, problems with aluminum walls, etc, and how to solve....

Along with what's already been stated, two layers of plywood in an area like that is not a great idea. If they needed more strength than one layer of 3/4" ply, they could have used 1" ply. The sandwich is a water trap; water gets in and cannot dry out.
Casey