Technics SL-23 TT opinions.. mated with Denon DL-103?

Sure, you have to look for tonearm effective mass, not every manufacturer will let you know what is the actual effective mass, but this is the most important as you can read in the Ortofon's article: "Cartridge and tonearm systems resonance frequency." 

Cartridge dynamic compliance and Tonearm effective mass - this is what you have to take in count. 

I can't give you an example of the turntable with high mass tonearm if your budget is $150.

The best high mass tonearm is Fidelity-Research, but it's about $1500-2500 just for the arm. 

But my advice is to buy some nice MM cartridge for your $150 turntable, sell your Denon DL-103 or just don't buy it. If you want to stay with Denon's "oldschool sound" and conical tip just look for DL-107 MM, designed for NHK broadcast (just like the DL-103). 

If you're looking for high-resolution cartridge with much better stylus (and much longer life-span, less record wear, better sound quality) look for Audio-Technica AT-ML150 OCC MM (Beryllium Cantilever / MicroLine stylus tip), which you may find for $400 NOS. This cartridge has medium compliance and it will put in the dust much more expensive cartridges. 

BTW I don't like Denon DL-103 and all the hype about it.   

That Technics is made for a mm cartridge. Most of the arms on those tables have a mass of 12 grams. If you use that cartridge the resonance frequency will be a high 16hz.
https://www.vinylengine.com/library/technics/sl-23.shtml

You do the math!!
http://www.mh-audio.nl/Calculators/RF.html

See what I mean?

I would just go check out the CL table. Ideally mount your cart and play a record. But if for some reason you can’t, then check it out. Pull the platter, turn the bearing by hand feeling for smoothness and slop. Check the belt for suppleness and signs of age like cracking. Replace the platter with the belt disconnected, give it a little spin and watch how long it takes to stop. Watch carefully when it stops- did it stop smoothly? Or abruptly at the very end? Or worse, stop and then go back as if the bearing were falling into a notch? Because that may be exactly what’s happening.

Handle the tone arm. It should move freely and without slop of any kind. Adjust VTF and anti-skate to zero and the arm should float and drift freely without ever sticking any one place. Bearing in mind this is an older budget level rig, and these tests check out, then for $150 you got yourself a $300+ table. I would take it and run.

The Denon weighs 8.5 grams without the hardware. The maximum weight for that tables arm is 8.5 grams without hardware. You might not be able to balance the arm with that cartridge. I'm just giving you the facts that you asked for!