There has been a lot of talk in this thread about the merits of mechanical vs electronic cameras and different metering options as well as a ton of other info. The OP stated his requirements as "costing no more than 75 US dollars on eBay? I don't need AF and probably only need Av exposure mode"
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With that in mind, there have been several great suggestions. The ME is VERY affordable. I just purchased a mint SE version in brown leather for $23. Now, it seems to be in perfect working order but I chose to pay for a seal replacement and CLA for about $80 because this camera was so pristine that I deemed it worth the cost for such a beautiful example. If my requirements were as the OP stated, I would look for an ME in the range of about $25 and pay for a DIY seal kit and have fun. THAT would satisfy his stated requirements with the best bang for his buck. Period. End of discussion.
I would not, at this point recommend an MX, KX, or K1000 or anything like that because he said that he shoots almost exclusively in Av mode. An all manual camera would be too much of a change and probably too much of a burden all at once. I WOULD however recommend an ME Super or one of the Program bodies, if they could be found within the budget. The reason is because one aspect of going back to film is that it slows us down and motivates us to think more artistically or creatively about each shot. Because of this, it is likely that the OP may soon realize the limitations of the lack of any manual control in the ME. I have had a total of four ME Supers over the past 30 years and NONE of them has ever given me a lick of trouble. The ME Super can be had for not a huge amount more than the ME, and still well within budget.
My prediction would be that the OP will either find the ME to be adequate for all of his needs and enjoy it greatly OR he will find himself wanting more and eventually gravitate in the direction of the MX/KX and/or the LX. I currently have the ME, ME Super, two MXs, and am targeting an LX, KX, SV, and K as my next purchases. Then I will go for the Canon EOS 3 to go along with my 7D2/5D3 kit and all of my associated L lenses.