I wouldn't buy amps secondhand either. You can't really tell if they still are in good shape. I am going to send my marantz for reparations this week. It has a cracking volume nob which causes ridiculous noise at low frequencies. Just needs a good clean most likely but you don't want this kind of crap from an expensive device. Older equipment, however, propably is of better build quality but amps do have a limited lifespan.
As much for speakers: modern speakers last (almost) forever. I've got a B&W subwoofer which is about 7 years old and it still works perfectly and looks great. It's active so the whole 'amp' statement I just made is kind of rubbish:P. The driver's conus isn't fixed by rubber but with somekind of other elastic material which is much stronger and won't rip apart that easilly. My floorstanders' drivers also look very durable. Old speakers' drivers commonly would rip apart eventually. I got the B&W for 525euros, let another guy pay the full sum is my philosophy^^. As long you buy quality stuff you'll probably be fine. If you personally don't like the idea I guess that's that but mechanically there aren't huge cons these days.
If I can give you some advise as much good speakers are conserned: go to a store and listen! That's about the only advise you need. I could tell you what my favorite speakers are but not yours. A lot of people live by the rule that B&W is the absolute best sounding stuff around but I found it perhaps the most overpriced stuff around. 'Most overpriced' however is difficult to say when you are talking about audio equipment. The guy I bought the sub from has a B&W 800 setup and made it sound like crap due room acoutics, positioning and bad sub intergration. Still thought his B&W's where amazing spending 10 times more money than I did and didn't have any treble nor tight bass. When I went listening for my setup I found most things said about speakers absolute rubbish. Everybody listens differently I guess. Whathifi.com said the Monitor Audio BW2 was the speaker of the year and it fitted in my budget so I was kind of settled on those. The only speaker that sounded any good to me however where from a relativelly unknown, and IMO underrated, company: ELAC. The speakers where a bit too expensive for me at the time but it was worth it.
When I got them I did some research(I didn't know the brand) on the speaker and it turned out they got an almost flat frequency response(on and off axis) from 200Hz. I guess I was pretty proud of my ears at that moment:P. The cheaper popular B&W's have a bit of a bumpy response (as do all headphones btw) but more importantly very bad of axis response. You'll hear literally 10 times less 2kHz @ 60degrees with the 685's (which is bit of an odd angle to listen at but is evidence of bad sound dispersion). B&W is very nice sounding though, just saying. ELAC is a brand that does all the design and production inside their office in Germany. They designed a very good driver like 8 years ago and they still use it: "don't change a winning formula". The costs of designing this driver are already made up for. This with the fact they don't advertise makes you only pay for the speaker not commercials and production costst. This compaired to, for instance, Monitor Audio who makes an 'amazing' new driver and speaker every year with lots of 'incredible' techniques. IMO my 700/pair ELAC's absolutelly made the 1200/pair Monitor Audio silver's look like a joke. The most expensive Monitor Audio's use sandwich membrane drivers just like my ELAC's and ELAC hardly talks about it...the irony. The moral of the story that the most 'bling bling' corparations are most likelly not the best. This illusion is sometimes created because people buy the marketing crap and don't listen propperly. At home they can't stop talking about "how amazing these new speakers are", at least that's the image I've got about how some very bad speakers seem to be amazing on the internet. I am very happy with my ELAC's but there might be better speakers for the price, I don't want to claim ELAC is the final word in price/quality ratio but you can do a lot worse in my experience.
With your budget you don't have the luxury to really look for 'your' speaker but you should go and listen to some speakers anyway, you'll be suprised about the differences between brands. What kind of headphones are you using? That might also give a hint towards your preferences. With 300$ you won't get hifi but some nice room filling sound could be achieved I think. Sony and Philips (in particular) make some pretty decent micro-hifi systems which are also interesting of you are not planning to upgrade your setup later on. I've listened to some 600/pair Mordaunt Short's which didn't sound much better than my sony micro-hifi with subwoofer and some homemade speakers IMO @ 350euro's but that says more about those Mordaunt's than anything else. It sounds rediculous but it's what both my dad and I my experienced. My dad has a Totem Staff, naim, arcam setup with 1,2k powerstrip etc. as a reference;). If you happen to realise secondhand is massive bang for the buck you'll can look into some much more intersting stuff. Realize it costs money to build a cabinet, 300euro speakers might have 150euro worth of drivers in comparison to 50euros in a 200euro speaker. This is not entirely true but in my experience 300-400euro speakers can sound at least three times as good as 200-300euro speakers. My dad's speakers again are at least three times as good as my speakers but they more than triple the price. I most admit it's a bit stupid to call a speaker 'three times as good' as if can be measured in such a way but it's just in a matter of speech. With your budget you should not be too picky conserning amps. I think using your tube amp would the best solution but apperently that isn't going to work. I don't know how good your headphone amp is but getting a good speaker amp is also a getting a good headphone amp if that argument stretches your budget.
That's about my philosophy and experience on speakers. Hope it helps