I was in a similar position and decided to go with the new k4. It is a big investment up front, but I am always impressed how well a high-quality pack retains value. Use it for 5 years and then sell it for 75% of what you bought it for. If you divide the difference between what you paid and sold if for by five years, it ends up being peanuts/year. I had a pack from a different brand that I beat the snot out of for 12 years and still sold for $275.
A used k3 would be tempting if it was in good shape, performed well for your body type and the price was right.
It really makes zero financial sense to buy a cheap pack. You suffer through five years of poor performance to end up with something that is worn out and has zero resale value. It ends up being more $/year if you do the math.