I have never heard of a saildrive main hull seal failing outside of a catastrophic event like landing on a rock and punching the drive into the boat. And I have searched hard, and talked to a lot of pros and others.
We finally got around to replacing our two drive hull seals last year, when they were 18yrs old. Both looked the same coming off as the new ones did going on. There was a slight amount of checking on the old ones, but nothing worrisome.
These are thick seals. If they did develop a leak, I suspect it would be self-limiting and very slow. I also suspect it would be easy to stop with common things on board - if stopping a slow weep was even necessary.
Changing them is actually pretty easy (if you are out of the water). Unbolt the engine mounts from the bed, unbolt the saildrive (remove the prop), support the back of the engine, and move the engine forward on the bed a few inches until the drive shaft is free of the flywheel. Then just lift the drive up and out, swap the seal, and put the drive back in the hole. Reconnect everything.
Not counting all the additional drive and engine work I did while I had the chance, replacing the first hull seal took me maybe 1hr, and the second was done in 30min.
But you don't get another chance to get this much access, so while the drive is out, it is a good time to replace the rear crankshaft seal, just because you are there anyway, as well as replace the input shaft seal on the saildrive - for the same reason. Both are inexpensive. Consider replacing the damper plate, although I didn't. This is much more expensive.
As for filters, the only thing that matters is that the threads are the same, the seal fits the boss, and the oil filter has a bypass valve. The canister size is inconsequential as long as you can fit it in the access space. Actually, the larger the better. Threads are pretty much universal in this filter size, the seal is easy to determine by just bringing it to a store and putting it against another filter, and a bypass valve is immediately noticeable inside the filter.
Cross references abound for these engines. I like Donaldson filters, that I get from filterspro.com for $3.50 each (oil). This one should work for your engine oil, but make sure first: P502016 - SFO0241 - LUBE SPIN DONALDSON REPLACEMENT FOR EXMAN SFO0241 - Filter Products