Gregoris - Hi and welcome to the Cruisers Forum - remember the two happiest days of boat ownership
are the day you buy and the day you sell - LOL...
I am no expert on steel hulls but I would not be afraid to own one. There are many members here with steel hulls and they are quite "proud" and happy with their hull
It is hard to diagnose exactly what you have going on but it does not sound great. First a question - If this 30 foot boat were in "great" shape how much would she be worth?
Also, I would recommend that you get a marine surveyor
experienced with steel to have a look. So here is my "amateur" opinion.
Any where there is rust at a bolted joint (the stancions for example) you should unbolt and grind away all the rust. If grinding away all the rust means you grind through the deck you need to think about fabricating replacement "plugs" that will need to be welded in place. If the grinding just went partially around the holes etc. I would consider building up with welding and filler wire and grinding to the original contour.
The bilge bottom plates sounds more severe - Are you talking about hull
plates? Or are you talking about bulkheads? In either case if you have "severe" rust in the bottom of the boat - you may have serious problems.
is very common but I think you are guessing, like I am, that the owner put the insulation
on to cover up some problems.
The only good news I have is that steel is very easy to work on. It is time consuming but basically anything that rusts can be cut out, fabricated and replaced.
Once again I would recommend getting a survey