Whoa.. that's quite the list.
95% of the mechanic/technicians you will be talk with are going to charge you based on "book time". Pick the task, look up the number of hours, multiple by the hourly rate. period.
Now, some shops will reduce two or more tasks when there is overlap-
A few places (like T3) will give you a real break and let you buy a day of labor and have a mechanic do all he can in that day. I'd venture the opinion that if you found an experienced Infiniti tech (like Jerry Tucker) they would have the knowledge to know what can be combined, and the skills to get the tasks done in (way) under book times. They'd be more likely to 'deal'.
There are plenty of posts on chains/guides and the costs. Ranges from $2500 at one dealer to $900 (I think) at T3 (the simplest guide replacement- no chains, no oil pan drop)
The under plenum hoses can be combined with this- makes sense to reseal the valve covers (if needed) and do the plugs and tune up. (Of course the belts should be zero charge if combined with the chain guides.)
If you are going to replace the steering rack (make sure it is the rack and not the hoses...) you might consider the engine mounts while it is apart- a bit easier.
One note: WHAT IS IN THE UNDER PLENUM KIT for only $185? There are vacuum, primary air, secondary air and fuel hoses. I did every piece of rubber, plus the heater hoses and brake hoses. It was NOT $185.
You've got no rear shocks in the list.
How is the cooling? If you need a radiator, it will be a free install if done when the radiator is out for the chains.
I'd prioritizer this as:1. Chain Guides (and all parts that are removed) radiator if needed2. Under plenum, Knock, Valve covers, tune up, fluids, cleaning, etc, etcThen, at your leisure

3. Suspension , brakes, misc other problems.
How much greens are you looking to spend at 'stop #1'? Might help refine the reccomendations
Hows the transmission? and the AC? I'd put in an AC system service- pump down the freon, check the compressor, repalce the receiver drier.
The rear seal is only accessible once the trans is out- $700 in labor- wait until you need a new trans unless it is pouring out
Welcome to Q ownership-
Ard