Need Help with 07 Avalanche purchase.

 
Hey all. I've got a friend who currently is selling his 07 Avalanche LTZ. It's a 5.3 with 167k with air ride suspension for 6k it doesnt have a cel but it has a service suspension and service wheels note on the dash. the wheels is because he has 3 bad tpms sensors which he is going to replace. But as for the suspension he said it has a blown shock in rear and needs a compressor. he said he will go half on the parts for the rear compressor and both shocks. can anyone suggest the cheapest way to get OEM style replacements as dealer wants 500 per shock and 500 for the compressor. I found the compressor for 160 a doorman and the shocks for like 280. part #23487283 and #22941896 or can anyone steer me another way for better prices? my other question is can I replace these on my own and are there write ups or is it best to let a mechanic do it.

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