Oh jeez! Looks like a series of bad luck... Question: Have you tried using a spring loaded chain tensioner? I haven't ever used one personally but I hear they are really handy and easy to install. But I'm not sure that would help so much if the issue is the hubs giving out. Maybe the main hub at the crank needs new bearings? I dunno. I had issue with my old MTB's cranks having sideways movement, and once I cleaned out the inside and replaced the bearings it was fine.

On a side note, it looks awesome and would be fun as hell on a downhill ride! I really want to convert my old mtb to a single speed!

It was replaced because the old owner rid it around for three years with zero maintenance and the old tensioner bent the quick release thus breaking the hub and had a stress fracture from being completely worn out. I'm guessing by the lack of maintenance done to it (nothing sounds like no maintenance like crunchy bearings), the bearings have eaten a trench into the shaft. Also the left hand pedal has masking tape between it and the crank, my guess the splines on the crank have been sheered off.

Oh dear god! Yea I also know too well the crunching sound that is neglected bearings... that's a shame. I just bought a cross bike for pretty cheap and the guy who owned it was riding it around like it's a beater! Like man... Bikes need some love too, right?

Singlespeeds seem awesome. Super strong rear wheel (since its not dished like a geared rear wheel to make space for the gears). More effecient, elegant , durable and lighter to boot. Definetly waaay less maintenance than geared bikes. Sorry to hear the previous owner was reluctant to do any maintenance what so ever though. She looks to have been quite the machine in her glory days (amazing deal for 50 bucks).... nothing a little time and money can't fix I suppose. Sounds like that chain tensioner caused all the problems though (unless they had given it a little TLC, slackers)

Sounds like you might need a new Bottom Bracket for the play in the crankarms maybe (hopefully it didn't mess up the bottom bracket shell of the frame). Could be hard to get a single speed rear wheel to replace it however.... I can't find them anywhere in 135mm spacing...trust me I looked. Custom builds only it seems. The hub you could get for that frame (with vertical dropouts) that would eliminate the chain tensioner would be a White industries eccentric Eno hub....https://www.benscycle.com/p-1522-wh.....FahaMgod9BQAHA............kinda expensive though but really cool.

I am currently building a singlespeed rigid mtb of my very own soon. I'm going to use it for singletrack, winter commuting and the Zombie Apocalypse. I have the frame (Soma Analog) and I'm excited that it has stainless steel sliding rear dropouts and therefore defeats the need for a chain tensioner or a fancy hub (you just slide them back and lock them down to tension it). I'm also going to build the wheels myself (first time). Paul hubs laced with DT Alpine III spokes into Velocity Aeroheat rims.

At the moment the only thing holding me back is my budget, hence why I re-used the old wheels sprocket and break disk and only got a new tensioner.

The only scary thing I'm worried about is my bottom bracket, as you said, it could be totally fried and will then lead to me doing a full welding job to replace it, did it to an old Raleigh Twenty so it shouldn't be any different.