Hi James, well, Pioneer's a good place to start. As are Sansui, Rotel, Trio...a bit of reading up wouldn't go amiss as you'll soon start to get a flavour for what to look out for. I probably know a little more about Sansui as their amps weren't in secondhand vogue a couple of years ago. Picked up my 717 for around £200, but it's easily the equal of amps at the £1200 or so bracket.
Don't write off the old receivers either - Pioneer's SX-1980 will set you back a handsome sum if you find one (ignore the one that someone's asking £7500 for currently on Ebay - apparently unused for 27 years. Terrific. Sounds like something that might be in desperate need of a service if you ask me...!), but an SX-1250 is sitting at £175 with 5 days left on it. Yes, that'll probably rocket!
It took me a while before I picked up my amp, so you might have to be patient. Really worthwhile when it comes round though. You just have to know when to stop...!