Two things to consider when purchasing a cheap airless sprayer.
1. Which one has the highest psi? Higher psi means less paint thinning. With a more expensive sprayer you may not have to thin at all.
2. Buy one with a metal gum not plastic.
When you are done cleaning up run a mixture of automotive anti freeze and water through the pump and gun. this will keep the insides lubricated and prevent rust. I store my gun in a bucket of the anti freeze mixture between jobs.
Good luck,
Craig