Here's My $.02 Worth.
First, you'll have to have an accurate heat loss calculation done on your house to determine how many btu's you need. Do NOT, repeat, Do Not size the new boiler based on the size of the old one - it's probably well over-sized. If the contractor won't do a heat loss (manual J or equivalent) on the envelope, send him packing. He's either lazy or doesn't know what he's talking about. Then measure the lineal footage of your baseboard elements. If it's standard 7" baseboard, multiply the total length of the elements (not the enclosures) by 500 to get the output capacity of the radiation. It will probably be higher than your calculated heat loss. If so, that's a good thing. The greater the radiation is over-sized, the more the supply temp can be lowered and the more efficient the boiler will operate.
Second, A mod/con will work great with baseboard because the outdoor reset, when properly set, will keep it condensing most of the time. It is true that mod/cons can be costly to repair, but so can cast iron boilers. I just came from one where we replaced the electronic flue damper, relief valve, gauge, installed an MBR and isolation valves and it was not cheap.
As far as a brand goes, it's far more important to get a competent installer than a particular make of boiler. If you choose a mod/con, the stainless steel down fired are the best: TT., Lochinvar WH, or ECR Dunkirk, Utica wall hung. I would not recommend anything that incorporates a Gianonni heat exchanger. They require a lot of maintenance and will fail quickly if fouling or scaling occurs. Buderus is down fired with an aluminum heat exchanger.
If you choose to go cast iron, the Buderus is unequaled, but almost any other would be good.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.