HVAC replacement estimates Burbank, CA

[Tosha S.]

Pardon the mini novel, but lots of details. Big expense, little experience w/HVAC.

I'm 11 mos. in to my buy and hold sfr and there's a cracked firebox on my rooftop 15-25 (lots of differing opions thus far) yr old 3 ton Lennox. 

In December my tenants wrote saying the Co2 alarm had been going on since November when they turned on the heat (10 mos after we bought the place and 1st winter). I filed a warrany claim and after a week was told it would be replaced with a 3 ton Payne unit (no new interior ducting). Total with permits: $6100

The ducting is old but the tenants thought the unit actually blew air well when working. 

I've had a bunch of quotes, but it's hard to tell what's a better deal/what I should do. I've had a quotes for a split system, too. Everything is expensive out here, but I want to make sure I'm in the ballpark at least. Here in CA you have to pay for Title 24 testing after a unit is installed if you go permitted route. It's pretty common to install without if that's wanted as it's $500-750 add'l. Great tenants, btw.

Quote: $8300 w/out permits for 3 ton rooftop Tempstar, moved about 6 ft for more airflow and 75% new ducting. Had another quote for $10,750 w/Bryant unit (10% new ducting).

Quote: $5950 for rooftop changeout no permits  w/ York unit 14 seer. Other ones included $6200 Payne 3 ton w/permits 6100 and 6950 for Bryant 14 seer no permits.

Quote: $ 8k for split system, new ducting, permits. Payne unit 14 seer, ADP condenser, Payne furnace. My brother will do electrical line for a deal. Total w/permits $8300. Unit goes within 5 ft of master bedroom so noise is a concern. I've been reassured it won't be as new units are quiet.

Part of me says the unit worked where it was so must be okay to put it back. One of the best local contractors told me he wouldn't warranty it and relocating it was best since new machines are so efficient they'll shut off w/out proper airflow. Perhaps he's right. 

Any advice out there? Thanks!