I worked in the composite plastics industry for over 25 years, so feel qualified to say a few things.
It is not difficult, I can teach you in about 2 hours what to do. After that, it is just practice.
There are a few rules to stick to (pun intended).
Everything needs to be dry, totally dry. Even damp can adversely affect the chemical structure of polyester resin.
Even your chopped strand mat needs to be kept dry, so no keeping it in a damp garage/shed. It will cause you problems.
UV light causes the polyester resin to cure faster, as does excessive temperature. So you have to pick a warm, but not hot, a dry, but not a sunny week to do it.
Then it is a matter of starting with a small area and working from there, don't be tempted to do more than 10 minutes work at a time. The OSB will absorb a lot of resin, so make sure it is well 'wetted out' before you put any dry matt on it. Then wet out the mat with a brush or roller. Don't be tempted to use a large brush or roller.
The mat takes a couple of minutes or so to absorb the resin, don't be tempted to try rollering (consolidating) it too soon, it will just not work. You can tell when it is working right at it goes almost transparent, rather than white, the noise changes as well, it sounds 'soggy'. Don't push too hard with the consolidating roller, that will just pucker it up.
Once the area is covered in GRP, you will have to sand it down and put a top coat/flow coat on it.
Sanding is a horrible job as it is itchy and makes your arms ache, but it has to be done right. Probably best to leave it a day to fully cure before sanding.
Then, when you paint on the top coat/flow coat, you have to make sure that it is adhering properly. You can tell when it isn't right as you will see tiny little pin holes. If that happens you have to let it fully cure, sand again, then reapply. All a pain in the behind, but it is what really makes a good weather tight finish.
Choice of materials is a hard one. I had my favourite mat 450gm CSM from Saint-Gobain, polyester lay up resin from Cray Valley and gelcoat (to make a top coat/flow coat) from either Norpol or Llewellyn Ryland. Pigments I usually got from Llewellyn Ryland too.
Hardener I was easy about, it is the same stuff generally.
Who you buy it off is up to you, but probably easier to find a local GRP place and see if they can get it for you, may work out cheaper.
Did I mention that everything must be dry, really dry.
If you are in an area that has a boat building industry, you may be able to 'find a couple of lads' that are willing to do the job.
I would charge more that £100/m2 so the price don't seem too bad to me.