Compare heating appliance efficiency -- the cheap way

A single largish room: Propane heat or heat pump?

Question

I'm almost finished converting my old two-car garage into a media room. It's got a 320 square feet footprint but has a tall 16' gabled ceiling, Walls are 10 feet tall so the total volume is around 4,200 cubic feet. There is currently no heat or air conditioning. To keep me cool, I can live with just a fan or two in the summer (the rest of the house is 180 years old and was never retrofitted with air conditioning, so cooling isn't paramount to me...but it'd be nice to have).

By the way, the addition is really well insulated. When it's around 32 degrees outside, a single electric space heater can make it warmish in 60-90 minutes and then maintain the temperature at half output.

But space heaters are unsightly and relatively pricey to run.

As I see it, I have three options:

Run a 5,000BTU electric fireplace (as needed) that I already own but haven't installed yet, and call it good.

Install a Rinnai propane heater of about 18,000-20,000BTUs. I've been told the heater will cost about $800, a 150-gallon propane bottle will cost around $1,000, plus the price of propane and professional installation.

Get a heat pump installed. It will supply heat and cooling as needed. I read somewhere that the average heat pump install (incl. the pump itself) costs six to seven grand but that's probably for a much bigger space...and I'm not sure if that's before or after federal and state rebates. Maybe I'd be best off with a heat pump if my out-of-pocket costs are somewhere in the $2,000-$3,000 ballpark. Not sure if that's realistic.

Thanks for any insight/advice!

Intelligent tinkering: Use the fireplace for now. But get a cheap off-market kilowatt meter (thirteen bucks from Amadump) and test the wattage and cost. Run the meter all heating season. Because electric resistance heat is highly efficient, thermodynamically speaking (no efficient losses like a furnace has), the wattage will also give you the estimated BTU consumption for the new room. Then run a simple cost analysis. Divide the installation cost of the two other options by the difference in estimated annual running costs between the electric heater and them, based on their specified BTU output, their efficiency, their fuel consumption or amperage draw, and the cost per unit for that kind of energy. This is just some simple math, and will give you the payback in years, ie, how many years it takes to pay off the installation using the savings. BTW, you can get DIY install heat pumps from Home Despot and other outlets. We picked up an off brand one from them four years ago now for $700, 18K BTU, still going strong.