Well, didn't you land a winner! £140? Nice and moreso if it's in good nick.
I liked the comment about it being hopefully good enough to live with £300 amps. LOL! I think you'll find if you do a few compares that you could stick a '1' before the 300 and you'll be a little nearer to what this amp's capable of going toe-to-toe with.
Big tip, spend another £100 or so and get it serviced. Don't be afraid to check if the engineer's familiar with Sansui and their issues - these amps don't crop up often (two or three a year on Ebay; compare that to Audiolab 8000As and the performance leaves them standing...) so it's worth making sure whoever's doing yours is aware of the problem spots to look out for. Check out the link below and select your amp from the homepage. The guy who did this knows his stuff, so you'll be doing well to get this level of servicing, but the author also describes what to look for in detail.
http://www.cdkands.com/index2.html
Speakers? Well, I'm biased with Mission, I think they made some superb speakers in their pre-IAG days and the 75 range - and I'd go so far as to say they must be up there with some of the best hifi of the last 30 years. The 752s still hold up well now, but depending on your requirements or room size, rest assured the 751s just get better the better the amp that's driving them.
In short, you landed a very high quality amp. It's an absolute joy to own, bringing life to music I thought I knew well. The scale, dynamism, detail and power music has can be breathtaking at times. Great phono stage too. Low level listening is the best I've heard; in our room, I can set it at the first notch on the volume knob and it's all there. Other amps struggle or find their sweet spot at a certain point on the dial, the 717 is happy wherever it sits and doesn't distort in a hurry; I've cranked it and the speakers are ready to go before the amp does!
I'll stop here I think! As I said, I think it's an absolute joy of an amp, I hope you think so too.