Depending on your time of visit I would not envisage you not being able to purchase tickets at the entrance. We used our hotel reservation booking from a well know on line Hotel booking site to order our tickets and was straight forward.

We bought tickets for the Sagrada Familia and did not buy tickets for the Gaudi House Museum. Inside the Sagrada Familia, there are elevators up some of the towers (6 people at a time) and there are elevators down from those towers that have elevators. A ticket must be bought for the elevators and there will be a time on the ticket for entry. The view from the towers is very special and the stairs are narrow and winding. If it is hot outside, it may be uncomfortable going up or down the towers by foot. The church interior is really beautiful and so very colorful. Gaudi had a wonderful imagination. The support posts and the ceiling vaults are so colorful. The Gaudi House Museum is in another part of the city (it is in Parc Güell) To visit the Gaudí House Museum, you must also have a ticket to Park Güell. There is an excellent place to go for information in the internet for prices and options for different tours. Go to TheBetterVacation site and take a look at the information there. Buying tickets in advance is the best idea. They sell out very fast if you don't get them ahead of time.

Either in the morning or late afternoon when the sunshine can shine thru the Color glasses and create a scene

Definitely no hats, or peaked caps, wearers will be asked to remove. Wear sensible clothing, it is a place of worship after all, not just another tourist attraction. Forget beachwear or immodest clothing, like most churches with high ceilings it will be cooler inside on a hot day. Did not observe any restrictions with footwear, sensible sandals should not offend. Beach footwear like thongs/flip[ flops would not be appropriate and possibly slippery on marble flooring.

I booked online in the UK. You download the Sagrada familia phone app. You book your tickets and an entrance time. Your tickets arrive in the app as a QR code to be scanned at the entrance. You can either hold the tickets on the app or transfer them to other party members phones - but they must be ‘live’ not screen shots from your gallery. Each ticket allows you to access the recorded audio tour so ensure you have ear phones to listen. You are then guided around the basilica via clearly marked number boards. We found the audio guide really informative and you can go as quickly or slowly as you like. so whether you’d benefit from a private tour I can’t really say I’m sorry. But, either way I’m sure you’re going to love the experience - have a lovely time.

I wore teva walking sandals and was not stopped. However, I was asked to remove my hat.

Tickets may be available at the Basilica, but they sell out fast. After not being able to get in lat visit, we bought our a few weeks early on line. The guided tour was 30€

Se intendi la torre centrale come quella della Natività’ ( che consiglio ) i gradini sono circa 500. Consiglio se non avete problemi di claustrofobia. Merita.

You should reach minimum 20 minutes earlier and chances are bright that you would be allowed to enter earlier. A private guide is highly recommended.

If I remember correctly- I downloaded app at hotel and then didn’t need internet on site as I just went from number to number in app.