Craigslist
Start witch Craigslist, stick name brand. Upgrade when you can scrape together a little more.
I started with an Ace 250, then went to a sovereign, then an Excalibur 2, now a GT's 3030,
The ace I bought on Craigslist for 100, sold it 3 months later for 125. Used my find to buy a used soveriegn.
I hear good things about the compadre, If I had to do it again I'd start there.
Stay away from the Walmart brands, they don't hold their value. Garrett, whites, minelab (pricey...) and tessero. All good brands.