That weld is common on Catalinas with double spreader rig, I think. It's on my boat. The double spreader allows less weight aloft. You should have noticed the taper. It's a performance enhancer. By contrast, look at a Catalina 34 with single spreader. The mast, at virtually the same height and close to the same sail area is seriously thicker and heavier. Look at a Catalina 36 to see how massive the mast can be and there is no taper at the top. Then look at every Catalina with double spreader and you will see a mast that is lighter weight and tapered at the top, well above the second spreader. I wouldn't worry about it unless the rigger finds a crack in the weld.
That round thing looks like a cover over the windex to keep birds off. But, as others have said, it's probably an antennae.
It's a good idea to inspect the rigging but at 30 years, I think I would replace all rigging & fittings regardless of fresh water or not. I told you to be prepared for it going into the purchase. While the mast is down, it will be easy to change out halyards, replace wiring or anything inside the mast that might look sketchy. Pay particular attention to the cable connections at the base of the mast and make any repairs deemed necessary.
Also, take a
close look at the stem fitting. The older ones, like mine, had a weak weld on the flange. There is an article in the Catalina 320 Owners forum that describes the potential defect. Catalina Direct sells a replacement that is far more robust and it was a very reasonable price in my opinion. I have all the confidence in the new stainless steel stem now that it is installed. The original one was sketchy adn I was glad that I read that article and addressed the situation in my first winter with the boat. I replaced all the bolts and was able to get them secured alone, but it would be a much easier job with 2 persons.
Removing it is tricky. You have to take the rub rail and the rub rail frame off where it bends around the bow (that means the rub rail will probably be taken completely off - but the aluminum frame comes off in sections). When I removed mine, the stem fitting needed to be bent ever-so-slightly to get it off when I didn't remove the rub rail frame. It made a difference because the new one will be custom-fit. You send the old one to CD for the template. When my new one came back it, the bend was a fraction of an inch off. (Photo attached) and it was because I had unknowingly bent the original while removing it. They returned the old one and I realized my mistake. It wasn't a problem for me after I took the new one to a machine shop and they corrected the bend for me (wayyyy to stiff to attempt any self-styled correction). You have to take the rub-rail off to put the new one on so you may as well start that way, rather than struggling to remove it with the rub rail on as I did.
If you change the stem fitting, while you will have to remove the rub rail, it's a good time to replace that as well. That was an easy job when the temp was warm and I had a heat gun to ease it into place. You shouldn't have to replace the aluminum frame unless you want to, depending on its condition.
You have stepped up to a seriously bigger and more complex boat than you had before. It should give you much more confidence in the difficult conditions you are more likely to encounter as your horizon is expanded on the Great Lakes. You should take into consideration that the rigging is critical and leave no room for doubt. I fully agree with
@FastOlson to replace the rigging, but I don't agree that you would get very far in price negotiation. Every seller is going to argue that the condition of the rigging is already factored into the price of a used boat. If you want new rigging, you pay either to install it yourself or when you buy a higher-priced used boat with new rigging. When selling, I would never reduce the price when the buyer suggests that I should cover that cost - no way, no how. Just move on ... So don't beat yourself up if it is suggested that you got taken. Every single boat that I looked at had the original 20-year old rigging and no seller was ever going to cave on that issue.
With a double-spreader rig, you are going to want a rigger to put it all back together. Why take a chance on your own? I wouldn't and I'm pretty supportive with diy projects.