You need to use 1 to 3 pounds of fruit per gallon. One pound is the bare minimum for slight flavor. Three pounds is a good amount for a firm and obvious flavor, and might be the maximum before it turns into more of a fruit wine with some malt in it, as opposed to beer with some fruit in it. If you’re not sure how much to use, 2 pounds per gallon is a good first stab. Then you can adjust future recipes based on your experience from there.
I agree with Loopie that you want to keep the hop additions light and easy. I would stay far away from any of the popular American varieties with strong citrus flavors. The best hop I think might be Glacier as it is a very mild hop with a slight peach or apricot flavor. But it is extremely mild – perfect for a beer like this. Keep the IBUs low. Maybe only 15 to 20 IBUs in this beer. Only use bittering additions. You do not need any late flavor or aroma additions at all.
Fruit flavor extract is not the greatest. The amount specified on the package is always way too much. However it’s probably okay if you use just a tiny bit to accentuate the flavors from real fruit. If you want to use extract at all, my best advice is, calculate how much extract you think you should use, then divide that amount by 3 and only use that little bit. Then taste the uncarbonated beer and see where you are at. You can always add a little more if you want. But just a little squirt is all you need, especially if you are mainly using real fruit. I still think 100% real fruit is the best way to go.
I would add all the fruit in the secondary. Boiling the fruit will release pectin that can cause permanent haze in the beer. On the other hand… since this is a wheat beer, it wouldn’t be such a terrible thing if the beer was cloudy! So I guess, try it your way and see how you like the result.
Another piece of advice: Plan to brew a small beer, original gravity of perhaps 1.035 or 1.040 max, because the fruit will ferment to give you a good amount of alcohol as well. Calculations on this are not so easy, but I’m thinking if you wanted like a 6% abv beer, a recipe for a wheat beer of like 1.040 should get you there because the added fruit will carry you closer to 1.060 and then to 6% abv. This is a swag and I might be way off, but I think this is the right ballpark. Someone with more experience might be able to tell you exactly how they do this.
And another thing: If doing this all-grain, mash temperature should be on the high side. The sugars in the fruit are nearly 100% fermentable, which will result in a dry, thin beer. So if you mash high, more residual unfermentable sugars will remain in the beer to keep the beer more “normal” – thicker and sweeter. I usually mash at 150 F, but for this I’d mash at perhaps 155 F.
Best of luck to you. Let us know how it turns out.