Converting to Gas/Indirect Hot Water Heater

New2this

I recently purchased my first home.  It is a 2 story home, originally a ranch up until 8 years ago when they built the 2nd floor, with a basement apartment.  3 showers and 2 laundry rooms.  The hot water does not work well in the winter.  When I first turn on the shower or faucet the water heats up very quickly.  The shower however will gradually get cold and the hot water does not last very long.  The problem is worse if the heat is turned on in the house.  It has been pretty annoying for everyone in the house so I am looking to fix the problem. Apparently the boiler is 20 years old and the coil isnt working very well anymore. 

 I want to fix the problem now but I am also converting to gas which will take place in about 12 weeks.  It was recommended to me to get an indirect hot water heater.  My question is would I need an indirect hot water heater anyways or when I convert to gas with a new burner would that solve the problerm?  If so is there a temporary solution until I convert? My house is approximately 3000 square feet.  Also, my boiler room is very small and tight and you can barely get around the burner.  I would have to put the indirect water heater in the adjoining room.  Thanks in advance for your responses!

SWEI

more info

Pictures of boiler and nearby piping/pump(s) would help, as well as nameplate data from the boiler.