I'm late to reply here, but (as someone who started shooting film Back In The Day, including lots of slide film) I would disagree -- I actually think a spot meter is a *bad* idea for slide film. What are you going to "spot" on? You need to find the element in your scene that approximates middle gray, and if you don't get that right, your exposure will be off -- and with slide film that means the lightness of the photo won't be right. (Unless you're scanning it, in which case the scanner takes its own exposure so it doesn't matter if your exposure isn't perfect -- and if you're doing that, you may as well save money and shoot color negative film.)
I think what you want is a center-weighted averaging meter, which evaluates the whole scene and grants a little more importance to what's in the middle. It's a much better method of metering, and the good news is that most cameras (including most M42s) have it. I shot slides for quite a while with two center-weight cameras (Pentax KX and MG) and did just fine, though I did sometimes bracket to cover my bases (especially at night).
You mentioned learning about metering, and what you need to learn is what situations will throw the meter off -- things like a very dark subject (say, a dark-blue car) or a very light one (a snowy scene). You have to look at what exposure your meter is recommending and think "Is this right?" There are a few easy checks to learn (which, unfortunately, I did not learn until I went back to film in 2019). One is the Sunny 16 rule -- it's a great guideline. (One of my favorite rail photographers, Don Ball Jr., never used a meter; he just looked at the lighting conditions -- what we now call Sunny 16.) Another is to meter off grass that is in the same light as your scene. Grass is pretty close to middle gray. If you have a fair complexion, you can meter off your arm. It's not perfect middle gray (and differs based on your skin tone), but it can tell you if the meter's way off.
Matrix metering is a great idea. Matrix divides the scene up into bits, meters each one, and decides what bits it should pay most attention to. Problem is, you will generally find it in cameras made in the late 1980s-90s. They are very automated and probably won't give you what you want.
Anyway -- spot metering is a red herring. Center-weighted is much, much better for what you want to do.
And -- if I may ask -- why shoot slides? Back in the day, we loved slide film for the way the colors exploded off the screen, and shooting it was a bit of a he-man macho thing because you had to nail the exposure. Now that we scan most film, I think scans of negs and slides look equally good on screen. From my own photos,
here and
here are scans of slides and
here and
here are scans of color neg; I think they both pop equally on screen. (You can see the film types at the links; note the night shot is cheap drugstore film.)
If you are trying to learn to get your exposure right, I would suggest a cheaper alternative: SHoot B&W and learn to read the negative density, which isn't difficult to do. That's the problem with color neg: That strong orange cast makes it hard to read the negative.
Honestly I'd still shoot color slide if it wasn't so flippin' expensive. That's why most of my color is digital and I do most of my film in B&W.
Nifty camera -- I inherited my uncle's 1000 DTL -- but if the meter doesn't work, they are apparently difficult to repair, or so was told to me by a Mamiya repair guy.
If you were to lose your M42 and spot meter requirements, I'd also consider the Pentax KM, which is basically a Spotmatic F with a K-mount. It was "decontented" to become the K1000.
HTH!
Aaron