The Airforce was fairly modern for its time, up to about 1927 it was one of the most modern in Europe (on par with France, if not by numbers then by quality of planes in use), after that it started lagging behind. Of course, it could never measure up to the biggest and strongest ones but it was quite adequate within the immediate region. During the 1930's the country fell unto hard times and so investing into modernization of the armed forces slowed down.
Nevertheless, when the April 1941 invasion came, the Yugoslav Airforce was capable of mustering some 800 first line planes, of which about 300 were of most modern types - Bf 109E-3, Hawker Hurricane IA, Rogozarski IK-3, Bristol Blenheim I, Dornier Do 17K, Savoia SM.79K, Caproni Ca.310, the remainder were older Hawker Fury, Avia BH.33, Ikarus IK-2, Breguet XIX, Potez XXV and others.
In addition to this, there was also a second line aviation which beside Breguet and Potez biplanes also employed pretty much anything that could fly in the country - trainers, civilian types etc. This brought the total strength of the operational arforce to about 1400 planes.
Yugoslav aeronautical industry has produced several domestic types but the most important aspect of it was license production of foreign types. Hawker Hurricanes, Bristol Blenheims and Dornier 17's among other types have been mass produced locally, often with local 'flavor' - small alterations and improvements. There have been interesting projects in development like upgrading Hawker Hurricanes with DB-601A engines, or the B-4 locally designed upgrade of Bristol Blenheim, and some locally designed prototypes - IK-5 heavy fighter/destroyer, Ikarus Orkan fast bomber, Zmaj R-1 bomber, Rogozarski R-313 reconnaissance bomber...
Having said all of that, what is important to realize is that there were serious political issues in the Kingdom of Yugoslavia at the eve of the war, which made a large part of its armed forces less than eager to fight to defend the country which they perceived as oppressive. If we take this into an account, and with the numerical superiority that the Germans and their allies already had over the Yugoslav Airforce, it is no wander the April invasion ended how it did.
Regarding the airbases, only 2-3 biggest ones had paved airstrips, most of them were grass fields. Especially the small dispersal bases from which the airforce operated in case of war. These usually had only the most basic facilities for refueling and rearming the planes.