Rob, if you’ve never built wheels before i wouldn’t start with stans! Allegedly they are easier to overtension (although I may be confusing myself with whati read about olympics rather than all stans rims).
355 erd is 540mm, it is 536mm for flow so at worst your spokes might be a smidge too long (somewhere between 1 and 2 mm given the angle the spokes leave the hub flange, geometry fans) It depends how much of the spoke is already in the nipple on your current build, the last thing you want is your rim tape getting pierced by too much spoke protruding from the back of the nipple. The rim swap bit is easy if time consuming (tape the new rim to the old wheel and swap the spokes over one side of the hub at a time), but for £120 of new rims I would be getting someone a bit experienced to do them as the lacing/rim swap is just the easy bit….