AIf you want a passable GF cake, any GF mix will work, a single tier should be fine. Taking GF cakes to the next level requires a significant investment in R&D and is probably overkill for what you need.
You will need to make sure you carefully sanitize all your surfaces and equipment (anything porous should be considered permanently contaminated) and follow a strict process for baking, decorating, and storage to make sure there is no cross-contamination with gluten. This includes not taking out or working with any gluten-free ingredients for at least 24 hours after flour has been used in the kitchen. You'll also need to carefully check ingredients to make sure there is no gluten or anything else that may have been contaminated with gluten.
If you really want to make a quality GF cake, you have to invest in several different flours and xanthan gum. Some of the all purpose GF flours out there will not rise well and have bean flour in them, which will give a bean taste.
If you want the flour mixture and recipe I use, just PM me. I bake GF for myself and several of my customers and they love the cake.
If this is a one-time thing, buying all the flours to make the cake from scratch would probably be cost-prohibitive. The Kinnikinnick cake mixes are pretty good, and Betty Crocker even makes decent ones (Amazon). To make it from scratch, check out the Land 'O Lakes website. Their gf flour mixture recipe is the best I have found, and their gf yellow cake recipe is a treasure. I make the flour mixture in large batches and use it for almost all of my gluten free cakes. This week I used it to convert an orange/poppy seed recipe to gf and got rave reviews from my customer.
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