Hot water boiler installation. Where to start?

[Generic Eric]

For the record, I think the OP is talking about heating the house...if not, ignore the following.

I am having boiler issues myself.  Its an older unit...maybe 40 years old or so.  They don't make parts for it anymore.   The last service man that came said the installer (a different person) never intended to come back and servicing this would be a tough job.  Pipes are plumbed this way and that...loops are interconnected strangely and the supply doesn't fill it for ***poo***  Oh, and it leaks but slowly.  It also makes a buzz and he says one of the "contacts" are stuck together.

It looks easy enough to  plumb this to that, and remove and replace an old part with a similarly functioning new part.  But none of that address the efficiency.

However, I don't think that is the OPs question. 
Where to start would be:

• Do you want an oil, gas, or electric boiler.  Do you have to have a boiler now because your current one went kaput?
• Does your utility offer off peak rates? 
• Is this your only source of heat?  (we have a fireplace that would heat the house, albeit, unevenly.)

There are so many places to start.

I think a plumber could handle this without a problem.  I am not quite a plumber.  As one person mentioned, there maybe some parts that you can't get without a contact from a distributor.  If not a plumber, then you have to be at least well read and know that parts are required.  *shrug* maybe I haven't searched thoroughly enough, but I haven't found all of the names of the parts....

Best of luck to the OP.