What is it you are wanting to test ? Voltage outputs ? Current draw ? Continuity ? Resistance ?
Pretty much any low end meter will do all of these. Some cheaper meters will only measure current in small amounts - 200 to 500 milli-amps. I picked up a digital meter at my local big box store that will measure up to 10 amps of current for about $20.00. You will want a meter that will measure at least up to 5 amps.
If you are wanting to measure DCC track voltage, no digital meter will measure it completely accurate, although the AC setting will get you pretty close. You will need an actual DCC volt-meter if you want accuracy.
Mark.