Sizing modern machines to replace 720's on V4R5

My company is running JDE 7.3 on old equipment and an old release of the OS. We are finding this situation increasingly expensive and risky. Hardware support is very expensive and software support is either expensive because of our software tier or not available. We want to move the application to modern machines running V5R4.

What approach should I take for getting a pair of machines sized and priced? We have a big enough budget for two 520 P10's. Any recommendations for hardware vendors are greatly appreciated.

We have (basically) two of the following set up as a replication pair:
9406-720-2063 w/ 1503 interactive feature (P30, 800 CPW, 'Enterprise Edition' I think)
3 GB memory
5x2748 RAID w/ 26MB write cache
1 TB at 40% usage (4X6713 9GB, 60X6718 17.5GB 10kRPM)
OS/400 V4R5

I gathered disk-busy percentage statistics at 5 minute intervals for a month and found that we exceeded 15% on any given arm only 30 times (to a max of 23% once). Our peak user count was 51.

We rented a 520 for an initial feasibility study and have found the performance very satisfactory at least for batch jobs. We ran the biggest batch jobs we have against our production data on this machine and found run times decreased significantly in spite of far fewer arms (which blows my mind).
9406-520-8953 (P10 2400 CPW)
8 GB memory
1x5709 RAID w/ 16MB write cache
0.5 TB (8x4327 70GB 15kRPM)
OS/400 V5R4

If we buy used machines, how do we acquire the LPP's beyond the OS? What else should I consider? Many thanks!

Phil