Huw, Besides the IYHA hostel someone already suggested you, there is another one I know of, but I don't know if it's good or not.
http://www.geocities.com/thetropics/cabana/7617/
english.htm They are farther than 200m rom the beach, though. I would say they are 500m. And with the current exchange rates, even some simple hotels would be very affordable. I really don't know if it's possible to go from Belem all the way upriver to Peru. A return air ticket from Rio to Belem costs something like US$500, but you can get it cheaper from abroad. There used to be an air pass that costed US$300 giving you unlimited travel, but it had to be bought abroad - I don't know if it still exists, check with Varig Airlines. You don't need to bring reais, you can exchange dollars when you arrive.
Maria Adelaide