Is $11,000 to replace existing rooftop HVAC unit a fair price?

I have an ancient rooftop unit that provides heating and cooling via ducts. The heating is gas, and the cooling is electric. The heating stopped working last winter, and the cooling is failing. I want to replace the whole unit with something new and go all electric. The HVAC company quoted me at least $11-13K, and they said they would need a crane to lift it in. I don't have that kind of money right now. I'm a pretty handy guy who's rebuilt multiple houses, so I'm thinking maybe I can find a solution myself. My ducting is already in place. I'm wondering if there are single units that have the A/C, furnace, evaporator, etc., all in one box I can just put on the roof and connect ducts to. If not, I'm wondering if the furnace part can live outside, or if I'd have to build a box for it. I'm trying to find the simplest single unit solution where I basically only need to rewire it and attach ducts. Ideally, a box that I plug in one end, and out of the other end comes both heating and cooling that I can just then duct into my existing system. I would not think twice about using the professional HVAC company I've been using, I just can't swing $13K right now. On top of this, the hassle of getting a city permit to block the street for a crane plus that cost means we're looking at $16K+ before it's all said and done. If there's a unit I could get up there myself, maybe modularly, I'd be happy to hire HVAC guys to hook it all up. One problem with the current unit is that the plate has faded in the sun, so there's no way to find out what unit it is or who manufactured it from the outside. Apparently, it has a cracked firebox (or maybe heat exchanger?), and that's why it won't fire up. The HVAC guys said it cannot be repaired, and the whole unit has to be replaced. I doubted that, as very few things can't be repaired or have parts replaced. I posted about this on the HVAC board here about 6 months ago and got shut down immediately. But that would be the ideal solution, to just fix it. If I can find the exact model, I wonder if the firebox can be replaced. I'm going to go up there, take a few panels off, and see if I can find a maker or model number on the inside somewhere.