I noticed somebody bought two of the LM's earlier from him.

Capacitors are rusted but fixture still works. I am wondering why two capacitors (apparently series) in a small 175 mercury fixture?

I bought myself a new volt meter with capacitance function so I can test all the caps in my lights before I use the .Besides, if you're concerned about the caps, install new ones . You get the piece of mind knowing that they're not shorted and won't leak PCB's or anything else everywhere .

I've never looked into replacing capacitors before. Isn't it difficult to replace them because they make like so many different rating capacitors and you have to get the exact one?

You can find the rating but the prices sometimes are way out of line. I have not seen two capacitors in parallel like this in a small wattage fixture before. The schematics are worn off the interior surfaces.

My brother found me the cap I needed for the 700w OV-50 from here : https://www.alliedelec.com  The new cap + shipping was like $15 or so .

...you have to get the exact one?

Yes. The micro farad needs to match with in 5% +/-. The voltage rating needs to meet or exceed.