Is the FHW Boiler in the same area? If it is, you could put in 20-30' of baseboard and cut into the return from your cooler areas like bedrooms so you get some heat down there. Since it's 50* all the time you aren't adding a lot if BTU to the boiler. This should cost maybe $300.
$2500 seems way out of line for a zone unless there is a lot of new piping and new manifolds and circulators involved, and you can buy a lot of electricity for that amount of money.
I have a office in a detached garage, 850 SF and have been heating that with a Monitor kerosene heater for years and considering changing to a split system heat pump for heat and cooling too when the Monitor dies, and seems these units are going to cost $2500-3000 plus installation likely nearly $4.5K but annual costs would be lower than the kerosene.
If you need AC and this can keep your house cool too then I would go the $5K route and with the most efficient units there is a tax credit too.