Buying Truck Need help!

Ok, first of all I have been driving for 18 yrs now with my familys company, all we have ever had is macks, my last being a ch613 with a 7/460. Heavy set up, god I miss that truck. we had to close the doors on the company a couple of years ago, I signed on with a regional FB carrier in the Jersey area, they screwed me for the past year and a half, so i am going back on my own, it is what I know, and actually have work. (i know sure I do)... Here is the deal Macks are great, I can't buy one, first the resale is good and I cant find the exact one I want used, So I have shoped and found some decent units, but being a Mack dummy, I know nothing about other engines, all I do know is the truck I am in now with this carrier (this is my last day) has the MBE 4000 Mercedes and has been on the hook more than on a load, so knowing this I dont want a 4000, all the trucks I am looking at have either Detroit 60 series 500 hp so they tell me or cat, i am in the 300k range in milage and am looking to spend about 40 to 45 k. A mechanic friend of mine(works for penske) that could be bad or good tells me the detroit egr model is a good engine and will do better on fuel than the pre motor, but reading round here, I see that might not be true, I have a line on a 05 kw with a i think they said 435 cat but i might be wrong, but the dealer is pushing the century on me with the detroit, he can give me a better warrnty than the cat, I know cat's have a good rep but all this pre post crap is killing me. Please help before I go and buy a 79 R model with a 40" bunk, at least Iknow how to fix it.. LOL Thanks in advance.
Be Safe!\Dave