Diane, I am sure you will get many excellent recommendations. One suggestion that I would like to make is that you contact Pat Hendricks Sensei at Aikido of San Leandro
(CA). In addition to having her own well-regarded uchideschi program, she has participated in the Iwama, Japan uchideshi programs of Morihiro and Hitohiro Saito Sensei for many years. Hendricks Sensei is a wonderful technician and person, and if you are serious about this undertaking (and it certainly sounds like you are), I know she would be only too happy to offer valuable insights into your options, and help you any way she can, as she has walked this very path herself.
On that note, I'd go right to Hitohiro's Dojo in Iwama and experience their uchi-deshi program, which ranges in commitments from weeks to months. His uchi-deshi program is well established.
When you return to the US you could take up residence at Pat Hendrick's dojo and continue in the same tradition.
Pat is a wonderful teacher and a beautiful person. She loved Saito Sensei and he treated her well. It shows through and through!