if they feared an oncoming war they should have got as many WWI surplus planes as possible.
Surely you jest. As polmed pointed out in her piece Poland had a far better resources to rely on then the flying coffins you speak of that were domestically designed and produced. Here's something for you to ponder on and a good parallel to what you have proposed, can you imagine what would have happened if Americans solely flew P-51 Mustang's against the Soviet made MIG-15's in Korean war? Besides that's not the real number of overall Polish air assets at the time either, merely a number used against the Germans in September campaign. There's a lot more factors at play to fully operate all the assets you have. Can you imagine the number of personnel needed for each plane to keep it operational?
A lot of Polish military personnel as well as equipment was moved to the Romanian corridor including planes reflective of the overall strategy of Polish high commend and perhaps their foolish faith in their allies and the confidence that the agreements actually made and signed would be honoured. Perhaps those worthless agreements in itself were just a casualty of another worthless piece of paper or propaganda if you prefer hailed as peace in our time by Chamberlain had something to do with that. When Poland finally fell Romania received 40 PZL.11's, 14 PZL.7's, 31 PZL.23's, 27 PZL.37's and 29 various other types of planes claimed by Germans as spoils of war given to Romania alone not to mention 130 planes that were taken over by the Soviet Russia and all very much still usable and useful for them to take, not some junk-yard surplus you speak of either.
During the September campaign Germans lost 247 planes over Poland and about equal number were damaged, so overall a loss of 333 planes on the Polish side vs. 247 on the German side is no small victory for the Poles considering what's hailed as technological superiority of the German Luftwaffe. Keep in mind that 44 of the planes lost by the Germans were 109's and 110's never to be used on the western front and a technological marvel in any western mind vs.25 Polish planes that could actually face them as equals in combat. Mind you Germans had 2700 planes over Poland during the September campaign so those numbers speak for themselves as to their so called great victory over Poland, with the help of Soviets. Victory? Perhaps, but with very heavy losses in my mind and short lived victory at that. Nothing to brag about, in my book one has to enjoy the fruits of victory in order to claim it and unlike the Soviets, Germans simply did not.