I've found a model on a site that I'd like to use but it doesn't come in a Blender format, so having never purchased a model before I'm just wondering which format I should buy? The formats available are .3ds, .dxf, .obj, and .max. Are they all essentially the same? I've seen in the Add-ons that these can all be imported, but I don't want to waste my money on one that won't work properly etc., so is there a type that's known to work better?
Most stores offer all file formats for download without restriction once you buy a specific model.
If that is not the case, then DO NOT buy the 3DsMax (.max) file, since it's a closed file format specific for AutoDesk 3DSMax and you will not be able to open it elsewhere.
DXF is generally more geared towards CAD-Like applications (like AutoCAD or FreeCad or LibreCAD alikes).
If you really have to make a choice either go with the .OBJ or the .3DS file formats, which are some of the most common and Blender as native official importers for both.
OBJ is generally regarded as a more modern and advanced file format, it is well supported in most 3D packages (in case you need to open it elsewhere in the future) and also supports the most features compared to 3Ds which is more basic and limited, so I'd say OBJ is likely the best from the bunch.