Given the resurgence of cassette and walkman's (prices on e-bay are constantly increasing) I think it might be useful to signal to potential buyers all those models which nowadays are hard if not impossible to restore or repair. My suggestion is to stay away from Sony WM-EX49 and derivatives (mine is actually a WM-190); those walkman's are actually very nice in the exterior design but it is very hard if not impossible to make them work. SONY WM-EX49 and derivatives problems: On the electronic side, the PCB is very thin and needs extra attention with soldering iron; there are several surface mount capacitors which have either leaked or degraded. On the mechanical side it is even worse: lots of tiny parts and gears which tends to break with time (the usual shrinking plastic); some gears are clipped and can replaced if you find a good one, some others are riveted and cannot be replaced; on top of that, the clutch might have stiffened or broken for good and the belt has very likely melted into black goo. Buy it only for display, even working exemplars will fail shortly.
I would avoid WM-150 I have several WM-150 including one in brand new with all accessories and some used. but i will not buy another one. This model is very much vulnerable to a gear crack that is difficult to repair. @bub posted a topic about how to fix it but still it is not a simple task.
In addition to the WM150,550C,170,190,EX49 Mechanism (which can be very good sounding when fixed): Panasonic RQ-SX 2 belt Mechanism, or AR10. Bad clutches, biodegradable gear, terrible rubber tire capstans. Aiwa TP-S30, Early models like J1,P1, etc: Nightmare. Aiwa Mid 90s 4ZM Mechanism: You need very good pinch rollers for the capstan take-up torque to work properly. Not the best (safest) tape handling. WM7, not easy to fix if the nylon gears shrink, or crack. All late 80s-mid 90s Aiwas, if you can't solder. Not for the faint hearted to re-cap, but usually great sounding when fixed. However, if you learn to fix them, the 150,4ZM, Aiwa Alpha,0ZM, types are fantastic bargains and great sounding. That's why I like them. But not for people new to Walkman Repair. I'd suggest WM-EX1/600 series mechanism for beginners.
Those were actually TOTL players of the period, very slim, auto-reverse, semi-automatic tape transport, "EX" head (for what it actually meant), big brass flywheels, metal casing, ... but they were plagued mainly by two things: typical Japanese over-engineering and (very) bad choice of materials/components. I would add to the list some Philips walkman's (sorry i do not have the model numbers) which used that typical orange translucent gear: that looks like planned obsolescence as those gears were made with a material that simply disintegrate with time rendering the walkman useless.
