What to look for in a Model 33 Teletype - a newb's buyer's guide?

[drlegendre .]

I've gone off the Deep End and now I just have to re-have a Teletype model
33.. I feel awful about what became of the junker I bought for $15 when I
was a 13 year-old and wish I still had it today, so I could fix it and put
it to use.

So what should a mentally-challenged guy like myself be looking for, in a
Model 33 - preferably an ASR model with the paper tape reader?

What does and doesn't go bad? Are there any serious game-changing,
show-stopping or buzz-killing issues that effectively render one of these
units as a parts donor? What differences exist between the various
ASR-model units, as they progress through the years? What does one look for
- and more importantly, what to avoid?

And one more - how do you differentiate a 20mA current-loop model from a
'standard' RS-232 model? Can you tell from the rear panel, or do you need
to go under the hood?