There are a number of factors one needs to take into consideration when buying a metal detector.
As there are many different types of metal detectors, probably the first factor to consider is, what are you going to use it for? Of course, in this instances it is for the detection of gold. Even then one can get metal detectors that specialize in detecting precious metals, or one can get a more general one if one is going to use it to detect other things such as old coins for example.
Then there is the consideration of who is going to use it? Is it just yourself or perhaps the whole family? If children are going to use it then one might need to consider a shorter pole and a lighter model for them. Children also seem to pick up on the technical aspects rather quickly these days (after all, who sets up your video or CD machine) so that tends not to be an issue. One can also get a kids metal detector.
If you are a new user then perhaps a simpler model rather than the more sophisticated may be suitable to start off with. There are hundreds of different models to choose from and each have their own instructions on how to use them. The question then is, which is the most suitable for you? Asking a friend will usually only elicit the model that they prefer and have found suitable for them. Most metal detector users tend to stick to one model or, at the best, brand of metal detectors and so have little of any experience of other brands and models.
Some brands of metal detectors are heavier than others. This may be important if the person using it is slight rather than bulky.
Some require more technical expertise than others. Some have a bigger or different 'swing' than others also.
As a general rule the more expensive the detector the more features and refinement it has. So how often do you intend to use it may have a bearing on your purchase. If you use it once a year just going recreational gold prospecting for fun, then a simpler model may suffice. But if you intend prospecting on a more serious note on a regular basis then a more sophisticated detector will be more worthwhile.
Of course your budget will have something to say about how much you can spend on a metal detector and that, again, will depend on the purpose for buying one. Whether recreational, or for a more serious hobby.
The main criteria is probably to buy the best metal detector you can afford which will detect gold at a reasonable depth and which is not too heavy or complex to use.
A good way of doing this is to write down the features you need based upon the points listed above and then list the metal detectors that fits these requirements the most. One can then check for reviews of those detectors and using them as a guide pick the detector that best suits your requirements and which is affordable for you.
Buying a metal detector is not as complex as it sounds and when you have been out gold prospecting with a metal detector, will likely get the bug. It is likely it will not the last gold metal detector you buy!