One thing I won't skimp on are rear derailleurs. Well, at least that much.
The cheapest ones are so flexible, I'm not sure they can be properly adjusted.
The newer ones are made with a much more rigid body structure.
I'm not quite sure what I'd choose. It appears as if you have 9-speed on the rear, with pretty large sprockets.
What brand of derailleur/shifters do you have? I believe most are Shimano compatible, except for SRAM or Campagnolo.
For Shimano, you are limited mostly to 8 or 9 speed rear derailleurs (changes in shift ratios for 10 and 11 speed).
My preference for derailleurs would be Deore, XT, XTR, 7000, 8000, 9000, m7000, m8000, m9000 (noting, of course different shift ratios for 10 and 11 speed which can mess up indexing).
I also like the Shimano "Shadow" technology.
When buying a derailleur, you will have the rear sprocket range, to look at (maximum teeth on the rear sprocket), as well a the capacity (rear large-rear small) + (front large-front small).
As noted above, there are three types of derailleur adjustments.
- high/low stops. This is what keeps your derailleur from over-shifting either into the spokes, or between the last sprocket and the dropouts.
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- Inline adjustment. This helps you maintain indexing. Somewhere along the cable path, allows fine adjustments of cable length.
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- B-Screw or similar. Near where the derailleur bolts to the frame, adjustment that does fine adjustment of how far the derailleur comes to the sprockets. You need to keep it as close as possible to the sprockets through all shift combinations without actually hitting them.