Need advice on updating boiler system

Hello Everyone, I could really use some help. I can't seem to get any solid advice from the local plumbers,or boiler people on what i should do. Enough preamble here is whats up.
I am In Rhode Island, and I have a old (circa 1940-1960's) Amercian standard boiler (not much on the i.d. plate. #0154T, and Series#2BTJ5). It supplys heat for 2 apartments and hot water for 3. I am thinking about relpacing it but don't know if i would do better to separate the hot water from the heat or keep it all together as it is now.
I also have issues with the hot water supply. 2 out of the 3 showers will run hot for a few minutes then turns cold. one upstairs sink has the same issue but all the others seem ok. I also have hot water pressure isssues in 2 of the bathtubs. Even with vavle fully open all you get is a trickle of hot, cold is fine.
Any ideas on where to turn?

Tell us what state you are in, and probably we can get better input from someone in your area. I am in S. Calif. Here we do not see boilers in residential heat, and it would make no sense to have domestic hot water served by a heating system that we really only want to run 2 months a year, in a cold year! Even if you are in a cold climate, it makes sense to me for domestic hot water to be separate from the heating boiler, but I am not up on the economics of those kinds of systems, so lets hear from the other guys on that.

The issue of not enough hot for all the apartments will probably be addressed by the new boiler or whatever.

The one apartment with low hot water flow, first check the valves or cartridges in the shower itself. Then look for a hot supply isolation valve for just that apartment which may be partly closed, or may have become clogged.

Yup, I think seperate w/hs would be better now that your doing all this new work.

Thanks Jimbo, I am in Rhode island. Also any thoughts on wether to stay with steam heat? and maybe switch water to natural gas? And if i do break it out, should i separate the first floor from the second? if piping allows? All my steam pipes are developing pinhole leaks.