Congratulations on a great looking and classical rifle in a classical and very useful African calibre! I have used the 375 H&H quite a bit and have had very good results with mine.
I can vouch for the fact that the Swaro Z3 is great glass. My hunting buddy put one on his 9.3x64, but I do have reservations about the Z3 on heavy kickers, not from experience, but from discussions around the spring system. As a result a have a NIB 3-9 Z3 which will probably be sold off soon.
I have had good luck with Leupold for this type of rifle, but it does depend on what you intend to hunt. I have used an illuminated 1.5-5X VXIII (just pre VX3) "Euro" with good success and it was hunted quite hard and it did many flights and many miles in Land Cruisers and never skipped a beat. I have also used the VXR in 3-9x40 quite a bit. I will be putting a 2.5-8x36 VX3 on my 375 Weatherby soon. Any of these are in my opinion reliable scopes on a larger calibre.
The 1.75-6 as recommended is also a good choice. If you like illumination the VXR range does have nice options including a 2-7X.
If you like Swaro and are happy spending the money the Z6 is a great choice if you can still find one. There is an extended eye relief model, which I have on my AR15 and the optics are great and offer a true 1X at the bottom with a massively wide field of view and a crystal clear 6X at the top end and little risk of scope eye.
For DG I like hand detachable mounts - as discussed recently elsewhere on the forum. Alternatively a 1X, or 1.5X, at the bottom works okay as well. Very little buffalo hunting is beyond about 80m and often 30-50m, aside from on floodplains such as in parts of Mozambique, so there is not much need for magnification. There is also a fixed 2X Leupold available through the custom shop that you could consider and it should be absolutely reliable and is very light. I would like one myself
. If you have a mixed hunt in mind there can be some value in the magnification for some of the other game species you may plan on taking.
The reliability of optics is of course far more a factor of a robust and reliable mounting system than those who have had scope failures care to admit.