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...after I figure out what the hell I'm doing"
Uh, I hate to tell you but I don't know if that's always possible, just pick up your wrenches and keep muddling along please .
I guess today's Scotsmen are not as thrifty (cheap) as my ancestors were, they'd have jumped at the chance to grab a lifetime vehicle like a manual gearbox W123 240D
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I'd stick with the factory TDC sensor as it makes a good RPM source .
I'm using a tiny tachometer like you show as a portable tune up tool , they're great .
I can't believe you're unable to find a cheap (under $100) decent looking tachometer, I've collected what I *think* (& hope !) are all the parts to add a factory VDO tachometer and clock to my 1982 240D ....
It's fun chasing the details, I don't really care what others think of how my vehicles are built, I'm a huge fan of the 'mix 'n match' philosophy .