The least expensive wireless system I would trust would be an Audio Technica ATW series, which start at about $500 without the mic.
A few basics - VHF (Very High Frequency) has greater range and better battery life, but is very prone to RF interference (will pick up A LOT of extraneous transmissions - even cell phones - and hums from electrical devices). UHF (Utra High Frequency) has less range and battery life, but resists RF interference much better.
Hiding lavs properly takes some skill. If you don't place the lav correctly you can pick up lots of clothing rustle, etc. You also have to hide the transmitter.
Placement is also key; if not positioned properly the sound diminishes substantially when the talent turns their head.
You have to be very careful gain-staging the levels, and if you are using two lavs make 100% sure that each lav gets its own individual frequency and its own individual audio track or you could end up with serious phasing problems; you still have to watch for phasing in audio post, so be sure that you edit the production sound correctly.
The Lectrosonics 400 series (about $2,400) and Zaxcom TRX series (about $4k) is what the pros use, usually with the CountryMan B-6 lav (around $300) which is the smallest lav on the market.
If you're going to try to get away with a cheap system your best bet is the Audio Technica W88 series. The sound is fair but rather noisy to my ears. You may want to replace the lav with something better. Stay away from Azden; I've heard too many nightmare stories.
If you buy used make sure that the frequencies you purchase are not already in use by other devices in your country. In the US we recently had Block B taken over for other wireless devices, leaving hundreds of sound guys with tens of thousands of dollars of useless wireless trans/rec systems.
