I never recommend people buy cameras from eBay unless they are willing to take the financial hit if the camera is not in the condition expected and the seller refuses a refund etc. For all the trouble that often occurs there, there is often very little money saved. I suggest buying from KEH.com or Adorama.com. You're buying from renowned dealers at these sites and you have a good return policy. Plus, if the camera you receive does not work out for you, they usually have another one that it can be replaced with so you're not running around eBay looking for another favourable auction in the event of a return. the Cameras on KEH and Adorama have usually been serviced before sale also. This is usually not the case with eBay.
I can't speak much about the Minolta X-300 as I've never handled one, but I will say that I've handled about 20 Minolta SLR's in my time and only 2 of them didn't have something wrong with them. Minolta SLR's have even gotten broken during perfectly normal shipping to me. I just don't have a very high opinion of their durability despite the excellent lenses and throngs of people who love them. Personally, I'd rather buy cameras with known track records of durability. All these things are old so we may as well buy the best that we can in hopes they will last long under our care.
I own and have handled several Olympus OM-1's and Pentax ME Super's. Both these cameras are very durable and capable of excellent photos in a variety of conditions.
The Pentax has the advantage of an automated shutter if desired. The only problem with this is that the shutter is therefore electronic and dependent on the light meter to work. If anything goes wrong in the metering system, the camera becomes useless aside from a single speed.
The OM-1 is fully mechanical and so if anything happens to its metering system, the camera will work just fine outside of not having a light meter.
Both cameras are very compact and quiet, making them great for carrying around on a daily basis, on long trips and just to get candid shots.
The Pentax is made mostly of plastic whereas the Oly is all metal. I prefer metal in terms of durability but the Pentax certainly doesn't feel plasticky in use and is well built.
Both cameras only go to ISO 1600, whereas larger SLR's go to 3200 (some to 6400.) In 1600 light, you may prefer the LED meter read out of the Pentax. You can clearly see the meter in the dark with this camera. The OM-1 has a swing needle that is a little more difficult to see in low light but it's possible. The OM-1's focusing screen is brighter and bigger than the ME Super though, so while you can expose more accurately in low light with the ME Super, focusing is perhaps more accurate and easy with the OM-1.
I suggest purchasing the OM-1n for it's more ergonomic film advance and motordrive capability. Only late model OM-1's, called OM-1 MD's, can use motor drives. You may not care about using one but it's a nice option and all Pentax ME Supers accept them. Personally, I think the ME Super drive (ME II Winder) is significantly more durable than the Olympus drive (OM Winder II) and if I were getting a camera to specifically use with a motor, I'd reach for the Pentax, easily. The ME Super's metal shutter can handle the abuse of the motor moreso than the OM-1's cloth shutter anyway.
If you want to do macro photography, the OM-1 is the must because it has mirror lock-up and you can install matte focusing screens which will work better with slow macro lenses which will black out the split screen. The Pentax does not even compete at this capacity.
Lenses for both systems are pretty much around the same price and availability range. I prefer Pentax lenses for overall sharpness but OM lenses for texture and bokeh.
Hope this helps! Good luck and enjoy!