I have a Merkel 140AE in 450/400 for sale on the EE now. If you read my ad, you will see that there were a couple of things that I didn't like about the rifle when I first got it. When you are going to hunt an animal that may have killing you near the top of his priority list, I think auto safeties are downright dangerous. I had a gunsmith in BC disable the auto feature on my rifle so the safety must be manually applied. Also, the original sights were off, by about 6" at 50 yds. I already had a trip to Africa booked, so I had no time to return the rifle to Merkel. The same gunsmith that did the safety also made a new front bead and a new rear leaf, so now the rifle is dead on at 50 yds.
With regard to load development, as stated above, it can be problematic. Unless one is very lucky it may take a lot of shooting to come up with a load that works well in your rifle. It takes a lot of ammo and for factory loads you're talking over $5 per round, so several hundred dollars goes down the tube pretty fast. I don't handle heavy recoil very well which is why I chose the 450/400. When working up loads and "breaking in" the rifle, I was shooting about 20 rounds per week because that was all my body could stand. This too, then, added to the time and frustration of load development. If Africa is in your plan, you also need to come up with loads for both softs and solids that shoot to the same POI, and this can complicate matters further.
I have vever had a functional failure of any kind with my Merkel. If you want to spend the big dollars, you can buy a better rifle, but for what they cost, I think the Merkel is a reasonable value. Based on my experience, if you buy a new one, you can expect to have to spend some money on mods and ammo, but you will end up with a pretty good double rifle.
FWIW, I bought mine from John Hipwell at Wolverine Sports.
What ever you decide, good luck with your double. I can almost guarantee that you'll be bitten by the bug.
Hugh