What OPK brand to buy?

So far I have been trying to predict O by using glow and monitoring CM. I'm thinking I might use opks next month to increase probabilities. Any rexommendations?? My cycles are somewhat irregular (25 to 32 days) so I will probably want to monitor O over a longer period of time (in other words, maybe some brands might work out to be way too expensive? Or maybe the price is worth it given I don't have a food baseline??)