If the place is air-leaky and basically uninsulated, doing the air sealing and insulation would provide more comfort than any heating system (short of radiant floors or something. You don't have to wait to re-side to fill the wall cavites with blown fiberglass or cellulose. Wall insulation doesn't have a big effect on the sensible cooling load, but the sometimes reduction in air leakage that comes with blowing insulation in walls will reduce the latent load, usually by quite a bit. Wall insulation makes a
much bigger difference in the heat load than it does the cooling load.
Talk to some contractors, get some quotes. It may be less than hacking in a gas furnace, and it won't tear your house up. You can fix any window flashing details that need attending to when you re-side.
BTW: What did the Manual-J come up with for a heat load? Did they spell it out room-by-room?
So your background power use is on the order of 2000 kwh (?), or is that during the heavy cooling season (seems more likely). At some point in the spring or fall you get a bill with very little heating or cooling use on it, and that's what we need to compare the 4500 kwh to. Unless you have a pool pump going 24/7 during the shoulder seasons your minimum bill is probably fewer than 1000 kwh, but whatever it is would be important to separated out the heating or cooling power use vs. the general power use.
The minimum legal HSPF of a heat pump sold in the US a few years ago was ~7 BTU per watt-hour, or 7000 BTU/kwh. (It's now something like 8.2 minimum.) If 3500 of the 4500 kwh was for heating, at 7000 BTU/kwh that's ~24,500 ,000, or about the same as 255 CCF burned in a condensing boiler or water heater. The mean January temp in say,
Alexandria LA is about 50F, so the average day is about (65F-50F= ) 15 heating degree-days, so over a 30 billing period that would be 450 heating degree days. 24,500 ,000/ 450= 54,444 BTU per HDD, or 2,268 BTU/degree-hour (which is quite high implying a really large or really leaky house, or a woefully leaky duct system.) The
99% outside design temp in Alexandria is +30F, which is (65F-30F= ) 35F heating degrees below the presumptive heating/cooling balance point, for an implied heat load of 35F x 2,268= 79,380 BTU/hr, which is just crazy if it's an average sized house, and yes, a 3.5 tonner won't really keep up with a load that high without resistance heating backup, which means the "as used HSPF" and heat load is actually a lot lower, due to the low efficiency of the resistance heating.
For reference, my somewhat tightened up mostly-insulated 2400' 1.5 story 2x4 framed 1920s bungalow with 1500' of insulated basement comes in at about 21,000 BTU/hr @ +30F. If the heating system failed a four 1500 watt (5100 BTU/hr) oil filled radiator type space heaters can fully heat the place @ 30F outdoors while waiting for repairs. (But we also have a wood stove, so...) Just about any 3.5 ton heat pump could fully heat my place even at +10F.
Buck a CCF isn't as cheap as I had expected- a lot of that has to be paying for the new distribution infrastructure. There are some places in the US paying only a bit more than half that much. At 102,000BTU/CCF (it varies depending on the mix), burned in a 97% condensing furnace delivers 99,000 BTU/ccf into the ducts. Normalizing to million BTU (MMBTU) it takes 1,000,000 /99000= 10.1 ccf/MMBTU, at a cost of $10.10. In a minimum-legal efficiency furnace (which could be more expensive to install if the exhaust venting is complicated) that would rise to about $12/MMBTU.
The average price of electricity in LA is about 9 cent/kwh. With a minimum legal HSPF of 7 that's 1,000,000/7000= 143 kwh/MMBTU, at a cost of $0.09 x 143= $12.87/MMBTU, not a dramatic difference. At an only slightly better HSPF efficiency it's still cheaper than minimum-efficiency gas.
If you want gas as a back up heating system and want to use a gas furnace, put in in parallel with, not in series with the package unit, and figure out how to vane-off the two systems when not in use. It's a hack, but it'll probably . Gas furnaces have minimum flow requirements, so do heat pumps, and the system really has to be designed to factor-in the impedences of both the cooling coil and gas heat exchangers to have a shot at making the flow correct for either. The alternative is to install an oversized hydronic coil in series with the package unit using a water heater to the coil, but even that has to be designed, not hacked.