Budget is a big factor, how much are you willing to spend on a detector, under or over $500? If looking around old workings, a high frequency detector may be the go, but if you are intending to find larger gold at depth, something with a lower frequency or a cheaper PI detector may do the job, alternatively, buy a detector with a high and low frequency budget doesn't extend to a PI detector.
If looking below $500 second hand, you will knock out detectors like the AT Gold, Lobo Supertraq, AT Pro, all three Fisher Gold Bug detectors, the White GMT/MXT/MXT Pro and Minelab Eureka/x-terra 705 or 70. The pick of the bunch for fine gold close to the surface would be Gold Bug2, GMT and Eureka, the rest will go deeper due to lower frequency of around 18kHz. All rounder detectors that offer more than one frequency may include the Eureka and X-terra 705/70, but the X-terras are limited to 18.75kHZ on their higher frequncy coil.
If under $500, the choices may include the Whites GM2/VSAT/GM3, first version of the Tesoro Lobo or Diablo 2, first version of the Fisher Goldbug, Fisher Goldstrike, Minelab FT1600, XT17000, XT18000 and Garrett Scorpion. Once again, the Whites detectors excel on small gold with their high frequencies, with best picks being the detectors with adjustable SAT controls (VSAT, GM3). The XT18000 would be a good buy if you can find one with the same freqencies as the Eureka (6.4, 20, 60kHZ). The Diablo/Lobo/Scorpion run lower to midrange frequencies for better depth, but less sensitivity to small gold.
Other things to factor in are current availavility of coils and spares, costs of repair services - will it be serviced here or in the US (or will the manufacturer touch it at all), whether you prefer manual or auto ground balancing, what extra functions you would like -signal boost, salt settings, iron or junk discrimination, weight, the number of batteries that you require each time you go out to keep the detector going, ability to hip mount, and so on.
Also if looking below $1000, some PI detectors come into play, including the Minelab SD2000, 2100, and the Garrett Infinium, which will all be good on mid to large nuggets at depth, but not so good on sub gram nuggets.
Looking abroad, you are starting to see some of the more expensive detectors come down in pricing, with infiniums, GB2, GMT, MXT, and Lobos going for around the $500-650 mark, if willing to shop around for second hand units on Ebay.us. Other detectors mentioned are cheaper again.
