Boiler size question

[stigedis]

So. Here is the situation. My garage is 30x40 with 10 ft. Walls. Well insulated all around. In floor heat, 3 x300' loops. Looking for heat source. I have been thinking on wall boiler or on demand hot water heater. Yesterday I came across a 3 year old prestige solo 175 boiler for sale from a friend of a friend. Comes with all controllers, expansion tank, pumps, basically go over and remove everything I want from the currently running system. Asking price is $1600. Questions are, worth even looking at? I believe it is overkill on size (likely only need solo 110). What consequences of running too big of unit? I have tried to look on the inter web. Have read it will create lots of extra humidity problems. It will change efficiency from 15-20% reduction during operation. So the attractive price may not be that great after all. Also what life expectancy on prestige units? Maybe I should pass on it........
Will on demand hot water heater set up even work? Uncle in law runs a quiet side water heater since beginning of season. He says that all I need for small shop?
Any advice out there? Thanks in advance?

 

[Jackfre]

Grossly oversized boilers are not good. Why was a a three year old system pulled? What issues did it have and what maintenance and parts were added? This thing is twice what you need. Some here use tankless successfully. I am not a fan. I've had to many service calls. Some tankless manuf do not allow their units to be used in closed loop systems. Some do. Takagi does and some here have had success with it.
I think that if you buy the right boiler with a good outdoor reset and plumb it primary/secondary you will be happy with the system and the performance.

 

[stigedis]

Boiler is being pulled as building will be torn down. Unit reported to be working issue free for the duration of its use.
Thanks for input.