Gate valves are pricey but i doubt you will find them cheaper in the city or at all (other than RC, which it where i got mine
). you can get them online for cheaper but by the time you pay shipping/tax/pst/duty (depending) and your time it might be worth it just to grab one and bite the bullet.
1) You dont want to restrict your emergency drain. If something happens to your main drain you want it to be able to handle all the water your pump is pushing. Also leave it out of your sump water so you can hear it trickling so you know something is a miss.
2) Use the primer as well as the glue as it makes for a better seal and cleans the dirt off. Both teflon and the Oatly glue are reef safe. Make sure you give the glue time to dry before running water through it. and when you glue give a 1/4 turn to ensure a good fit. Remember when you dry fit to leave a little extra in your measurements for when you cement as it will go a little further in then when you dry fit.
3) I have used the 90 degree barbs before with no issues. yes they restrict the flow, but try this calc and see what it really does to your flow.
http://www.reefcentral.com/calc/hlc2.php