ABS or PLA for structural printed parts? TL;DR
Is ABS a better material to use for structural parts of a 3D printer, as opposed to PLA?

I have been looking at various suppliers of printed parts for a Wilson II, on eBay.
One supplier supplies all of the parts printed in ABS, whereas another supplier provides the structural parts, which form part of the frame, in PLA, and the remaining (non-structural) parts in ABS. The second supplier states the following:

This kit is printed in two materials, the structural parts are printed on high-quality PLA to provide the necessary rigidity, and the others in premium ABS.

MJRice, who developed the printer, also supplies the printed parts, which are made of PLA.
Is PLA really a better choice for structural components? A quick google lead me to ABS or PLA: Which 3D printing filament should you use?

ABS is going to give your projects better structural integrity and will be more suited to mechanical use given the material can better withstand the elements, but it will also require specific types of printers and printing surfaces. On the flip side, PLA will give you more precise prints and better aesthetic quality, as well as more flexibility with printing conditions if you can do without the strength and resilience of ABS.

Another link, What's stronger? PLA or ABS?, also implies that ABS is stronger than PLA:

The strongest ABS is stronger than the strongest PLA

and

ABS is much less brittle than PLA

although warpage could be an issue...

I choose ABS for it's strength over PLA.  I would love to avoid the warpage/shrinkage problems of ABS.

Heat seems to affect PLA more than ABS (obviously, due to the lower melting point),

FWIW, I know a guy who had a reprap using PLA-printed parts.  He was taking his machine around to do demos and he left it in the car mid-day once.  It melted the parts enough that prints were coming out very poorly even after his best effort at recalibrating the machine.

and as I am in Thailand, my room gets up to 50°C some days (it is unbearable), I wonder if that would be cause for concern?
So, from the above nuggets of information, wouldn't ABS be a better choice? I am surely missing something, as both the second supplier, and MJRice, use PLA.
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.