Been messing around with the R7 for a bit and yeah, that buffer issue is purely a write speed bottleneck. Basically, the camera is dumping way more data than your current card can handle. If you want to stay under your budget, you gotta look for that V60 label. V30 is technically the floor for standard 4K, but if youre planning to shoot in 4K Fine or 60fps at the wedding, youll definitely want a V60 card to avoid any hiccups. Honestly, for a wedding where you cant exactly ask them to redo the kiss because your card lagged, I would grab a UHS-II card. The R7 has slots that actually take advantage of that extra speed. A solid mid-range pick that wont break the bank is the Lexar Professional 1800x 128GB SDXC UHS-II V60. It usually sits right in your price range. If you want something super reliable, the ProGrade Digital SDXC UHS-II V60 Gold 128GB is a beast and handles high bitrates like a champ. Just stay away from the cheap generic cards you see at checkout lines. Stick to brands like SanDisk, Lexar, or ProGrade and youll be fine. Just make sure it specifically says V60 on the front and you wont see that buffer error again... hopefully saves you some stress before the big day.

Omg I totally feel your pain! I have the R7 too and it is such an amazing camera once you get it dialed in. I ran into that same buffer full thing when I first got mine and it was so stressful trying to figure out all those crazy labels on the cards. You definitely do not need to spend 150 bucks to get this working right though! Before you run back to Best Buy, are you planning on recording the entire wedding ceremony in one long continuous shot? Also, are you going to be using the 4K Fine mode or just the standard 4K? Knowing exactly how you're gonna shoot will help me figure out the cheapest card that wont fail on you during the big day!

Same setup here, love it