You don't really get rewarded for finishing your dex but it's a nice little accomplishment. I mean I don't even have a sandslash and I'm almost lv 30. Atm I'm missing 6 kanto pokemon :dragonite, snorlax, golem, sandslash, muk and charizard. I have enough candies for a charizard but my highest iv is 78. Atm I'm walking with a 93 iv grimer.
Candies for the dex or for powering up?
From kanto I had 8 pokes left. 5 of them are evolutions. I have walked around 200 km for getting a machamp, 150 for getting a charizard, 30 for slowbro, 50 for magneton and electrode. And now I am walking for getting my first gyarados (only 50 km left), but after that I have to walk another 180 for an omastar and, 25 for dragonair (I had a lucky dragonite catch). With that amount of km I can max out my good IVs blissey, my two good IVs Snorlax and my defending dragonite, Evolve my third Ryhorn and see if I dont get another Megahorn one (yes two with megahorn, and in the times lapras was good, had a good Lapras.
So I really look your advise, candies for completing that hurtful 9 spots, or to power up my competitive mons.
I walk with the game arround 4-8 km a day, depends of my activities. (Not fat, just doing exercise in a real gym)
Answers
I think you should always focus on competitive mons, eventtualy you will be able com complete your pokedex by finding this mons in the wild. I walked with charmander, machop, squirtle and magikarp and i really regret. For what i can see you live in a rural dry area with no sea, lakes and rivers. Finish your gyarados wich is a competitive mon and go focus on the defenders. Try to look for places not so far from where you live that has a water biome, and take a day to farm in this areas, you will see how commom some of these mons can be, you will probably be able to evolve your gyarados and find a few of omanytes and dratinis.
I find it a difficult situation. I'm living in a biome myself that mainly used to spawn pidgey/rattata all day. I know it is a popular statement to say that eventually you will gather enough candies and it's true.
However, it may take a very very long time if you have a bad biome.
Long story short; choose your own priorities. I found my dex more important in the beginning and I didn't regret walking my grimer and starters.