Need pump advise. What pump will run a ro/di filter?

[Delphinus]

This is what I would do. I'd have the barrell collecting the discard water off the RO/DI with a garden variety sump pump. You should be able to pick one up at HD/Rona for $80-100. Probably even comes with a float valve. The pump just routes water to an outside rain collector barrell. Then install dripline irrigation style soaker hoses to whatever parts of the garden need water.

The problem with watering grass is you tend to need a large coverage area. A pump could run your sprinker but if the intent here is to be green, then really be green, you know what I mean? Let gravity do most of the work for you with the slow flow irrigation instead of using electricity to spread the water.

Those sump pumps are not something you'd want to run long term, but in this case it's (presumably) just lifting the water from one reservoir to another. Its duty cycle will be very small that way.