5 hp trash pump should work for 10' up and 25' away easily and run a 2.5 dredge when needed. ALWAYS buy bigger than your needs as you can ALWAYS turn a pump down BUT when maxed out your tinker toy you are tough kaka outta luck. 1 1/2"-2" at least. Better yet just get a real pump like keene,proline,nugget,gold king,precision and then your golden. John
PS-much easier to haul around a 3-4 hp briggs/honda with a much lighter pump that does much more than haul a extremely heavy cumbersome trash pump a as much bigger and heavier for less results. Costs are close as smaller does more especially in the back pressure,psi,gpm arena.