Once you get the air out of the system, unless there are leaks, you should never have to add any again. The system pressure must be within the specified range for the boiler. If it peaks too much, then you need to add water, the expansion tank is probably shot. The amount of minerals in what is probably less than 30-gallons or so of water should not require deliming, or if it does, not very often. You just can't disolve that much. Now, if it is constantly refilling because there are leaks, then yes, it could be a significant problem.
A new boiler is likely to be more efficient, so you'd have to do a cost analysis to see how long the payback period would be. There are often rebates from the utility company and possibly the feds if you buy one that qualifies, so take that into consideration.
Unless the system has loops that go outside, typically you don't need antifreeze in it (unless you turn the heat off for extended times in the winter). Adding antifreeze decreases the efficiency of heat transfer, so your boiler needs to be bigger or run longer to transfer the same amount of heat to your house.
I'll defer to the pros for your other questions....