Initially hire one off your local association …
Used extractors sell for quite a high proportion of new price.
So, buying used and buying at the right price SHOULD mean that you needn't lose money when you sell on (to upgrade, naturally).
This isn't the best time of year to buy - new or used.
If determined to buy new, sales and shows provide the best opportunity.
If buying used, this (peak extracting season) is obviously when prices are highest.
Capacity (by frame number) is one criterion - but it isn't the principal one.
Frame type is a subtle one. Manley super frames really need to be extracted in a radial. But even some radials don't take Manleys. So extractor choice might limit your frame choice. Then there's brood box frames - do you have the possibility of extracting them? (Particularly a question for users of larger frame sizes - like my 14x12s.)
The extractor type advocated by JBM really requires the assistance of a musclebound hulk (or two) to lift the large tank so that it can be drained. Mind you, others manage with their extractor on the floor, lifting it after each load or two to drain it. My extractor sits on legs, so a reasonable-sized bucket (and double strainer) can go under the honey-gate whenever I fancy.
I chose a motorised radial extractor - not so much for speed, more as
not being a test of my stamina. I'm pleased I did.
But start with your Association extractor. That's what its there for. And one of the reasons you joined.