I'd suggest the Olympus OM1n. It's a small and beautifully made fully manual SLR and was built for professional use. They're relatively plentiful, usually quite cheap now (or even free) and many were really well looked after although they are able to stand up to a fair bit of abuse.
I have a couple of them which I still regularly use and I far prefer them to the similar models from other manufacturers which are just bulky heavy and noisy by comparison. Olympus lenses are great and the OM system really specialised in macro to tie in with their production of microscopes and medical imaging devices. Another huge benefit of OM lenses is that they ALWAYS have a depth of field preview button on them, so you can instantly and easily see exactly what the effects will be of your chosen aperture, regardless of which body the lens is mounted to.
The only downside to the OM1n is that it's designed to use a mercuric oxide battery, and they're now banned in most countries. You can instead use a Weincell zinc-air battery which produces exactly the same voltage but which has a lifespan of just a few months, or use either the CrisCam MR-9 adapter or the Felix de Gruitjer equivalent device which costs half as much for those of us living on the eastern side of the Atlantic. If you live stateside I'd go for the CrisCam. The adapters simply reduce the voltage of a silver oxide cell to the 1.35v of the original mercury cell. The same adapter fits many other cameras and lightmeters which used the same battery, so you keep the equipment original and when it moves on you keep the adapter. if shooting on colour negatives then there's no problem at all, just use a lithium cell PX625 which is the exact same shape but has the wrong and unstable voltage (starts too high and constantly drifts down as it's used to being too low). Negative film has a wide exposure latitude, and a stop of under or over exposure is easily corrected at the printing stage. Note that the battery powers the meter only: everything else is clockwork so the full range of shutter speeds remains available. With a dead meter or dead batteries you can usually get a usable exposure by following the Sunny Sixteen rule.
Olympus also made the OM2n, a virtually identical model which had both metered manual exposure like on the OM1n but added the option of aperture priority automatic operation. You set the aperture to suit the bokeh and depth of field desired, and the camera sets the shutter speed for you. OM2n also boasts a superb Off The Film flash metering system (stunningly good but especially so when using single or multiple flashes for macro).The OM2n is also relatively cheap or free but it depends entirely upon (easily available) SR44 batteries for operation.
The letter "n" after each model number shows that they are later models with a few internal improvements. On the OM1n the only real difference in use is that the viewfinder has an LED to confirm when the dedicated flash has fully charged and which blinks to confirm correct flash auto exposure. The OM2n uses the LED in the same way, but is also fully compatible with OTF flash metering on the Olympus "T" flashes introduced at the same time.
There is also a later OM2 Spot Program, it's a superb camera, but uses far more electronics inside, and the circuit boards have had reliability issues as they age. Truly lovely when they work, but best avoided. They also weren't constructed to the same professional use build quality as the earlier models as by then Olympus regarded the OM3 and OM4 as their pro-spec models. Avoid the OM4 for the same reasons as the OM2SP, you're also unlikely to find it cheaply. If you get offered an OM3 then it's still regarded as one of the finest ever fully manual SLR's, but they're rare and expensive.