I think the starting point is to consider whether you want to use this exclusively in your garden, or whether you also want to be mobile and run your trap at 'remote' locations where no mains electricity is available.
If the former, you have the choice of running a trap with an actinic lamp or a mercury vapour (MV) lamp. If the latter, you will probably need to use a 12V actinic trap, which can be run off a battery, unless you own or are prepared to buy a generator.
MV lamps are the most effective for attracting moths, but another factor to consider in making this choice is whether you will annoy your neighbours with a bright light: MV lamps are very bright, and if this is an issue, actinic might be preferable.
Once you have decided on the type of lamp, you then need to consider whether you want to buy a ready-made trap, or make one yourself. Ready-made moth traps are bizarrely expensive for what they are, and if you have rudimentary DIY skills, it's quite easy to make a trap yourself.
I made a Robinson trap with 125V MV lamp, for about £100. This included pre-assembled electrics from Paul Batty. I think this is probably the best solution for budget-conscious garden use where neighbours are not a factor.