However, after the installation, one specific steam radiator is incredibly slow to heat up. 

The attic really doesn't need any significant work, I can tell people have lived in it before - It's got bedrooms walled in already, there are radiators up there, windows, etc.  

All the heat is done by old radiators with a 100 year old radiant heat furnace in the basement coated in asbestos that can't be removed because it's too large.  

I bought a really old 3 unit (each unit about 1300 sqft) and had radiator heat and window units.

Boiler system is adding  baseboard radiators with pex line would be my first guess  More than likely it had enough residual heat from above to keep it above freezing and so pipes wouldn't freeze before.

Does the house have forced air or hot water heating, and if so, where are the vents/radiators?

They don't express empathy when they learn your radiator blew and the bill for repairs was twice what the mechanic said it would be and then you lost your job because of a urinalysis but you're going to fight it because you've been clean for months and you're really stressed because your mom was just diagnosed with breast cancer and won't be able to support herself by working at the Kitty-Kat Lounge and besides that, you're not paying rent until the oven is fixed because it hasn't worked since I moved in five months ago...Talking to Tenants will eventually wear you down.

The house has one shared gas bill with a steam-powered boiler providing heat through radiators in only floors 1 and 2, leaving the basement and attic floors with no heat.

My property uses radiators for heat and window AC units. 

How much should I expect to pay to convert a 2 story townhouse plus a finished basement from radiators to a forced air system?