I don't know what you're intending to keep in it, but a 350lph filter would be FAR too small. I think you'd want a minimum of around 2000LPH. But, you're not asking about filters. You're wanting a return pump from your sump, right?
If you're going for really dirty fish or for very heavy stocking you'll want something really serious. You could send Dogmatix a PM, see if he's still got that UP6 for I think it was $180ish. That will give you somewhere around 4000-5000 LPH if your overflow and everything can handle it. Otherwise there are a lot of smaller pond pumps that should work well. Something like a Powerjet 2000 would get you about a 2000LPH turnover which would be a nice comfortable amount.