How long of a tour are you planning? I find with anything under a month, one pair of rear panniers plus the top of the rear rack is more than enough space.
.

• Tent, sleeping pad, sleeping bag in one pannier
• Cookware, tools, and other assorted daytime use items in the other pannier
• Clothes, toiletries, electronics, and personal items in the rack top bag.

​​​​​​Keeping your stuff themed in this way means you can take the rack top bag into your tent at night and leave the other two panniers on the bike. Saves a bunch of hassle everyday.

Ortlieb makes a good rack top duffle.