ABS or PLA for structural printed parts?
To answer the main question "Is ABS better for structural parts of a 3D printer as opposed to PLA".

The answer, unfortunately is it depends.

ABS has lower yield and ultimate strengths compared to PLA. This means that at room temperatures, ABS is weaker than PLA. However the difference between yield and ultimate for ABS is much larger than PLA, meaning that ABS parts will deform, noticeably so before breaking. This can be a good thing. If your ABS part is highly loaded, you may want to be able to inspect it for overloading. The plastic deformation that occurs in ABS may be sufficient for you to identify, and modify your parts before a catastrophic failure. With PLA though since the two strengths are quite close, your part would more than likely fail completely without any warning or deformation before it. So in terms of strength, it's a design consideration. Would you rather have a slightly weaker part that shows signs of load failure, or is a catastrophic failure acceptable. There are use cases where either may be better.

As Ecnerwal pointed out, PLA has a lower Tg (glass transition temperature) compared to ABS. If your printed parts are going on a 3D printer and aren't sufficiently isolated from the hot parts (print bed or extruder) then you may end up having some localized deformation. That being said, the extruder is generally pretty localized and likely won't cause you too many issues assuming you're using any of the already available 3D printer designs out there, same for the print bed. However by the sounds of it your 'room' temperature is a fair bit higher than here in North America, by a factor of almost 2! This could cause you some issues if you're printing parts in the high heat with PLA. Your best option would be ABS, but even with it's higher Tg (roughly 90C if I remember correctly) you may still run into issues. 

The warping and shrinkage issues mentioned in the question seem to be more about parts being built by a 3D printer. Once the parts are built warpage and shrinkage are essentially a non-issue unless you've heated parts back to within their melting temperatures.

Depending on where you're getting your plastic parts, there may be a third option. By the sounds of it you're looking at building your own printer. You may be able to get the STL files of the printed parts you need and then have someone print them for you in a stronger material such as Nylon. 

Assuming alternative materials aren't an option I would personally suggest going with ABS for your use case. My reasoning is purely due to the potential operating temperatures. While 50C is below the Tg for PLA it's getting very close.