Build website for raffle & giveaway

This custom site is to allow customers to purchase an online ‘ticket/tickets’ for a chance to win a prize which we will list.
 
 For example we will advertise a product and to win this product there will be 1000 tickets available at £1 each. When all tickets are sold we will choose the winner at random on a live draw and if their number is selected then they win the price.
 
 By UK law this competition has to be called a ‘game of skill’ so before you buy a ticket you have to answer a simple question (e.g how many wheels does a bicycle have? 1, 2, 3 or 4), the correct answer is selected (question will change on each competition) and then the customer will select how many tickets they want to buy. After paying at checkout they will be given their number such as tickets: 100,101, 102.
 
 This site has to be 100% reliable and secure, by UK law we need to be very sensitive with data and also make sure that all entries once paid for are then added automatically to a excel style spread sheet in numerical order, like below:
 
 Customer 1 - Ticket number 1
 XXXXX - ticket number 2
 XXXXX - ticket number 3 This excel chart has to be published online before the draw for entrants to see their number again and confirm they are entered.
 
 
 Payment will be via a credit card company and fully integrated. This website will be an additional to our current one so we already have a large customer base. We expect a high volume of visitors to this site so we also need to keep in mind the server and that it can handle a large amount of visitors. 
 We have examples we can send for similar sites. 
 We need a countdown for how many tickets are left (e.g 100 left from 500 entries).
 Once all tickets are sold the entrants will get an email with their entry numbers on. 
 It is really important that this feature is easy for admin to use (change question each time etc) and also very user friendly, it should be a simple 2-3 step process. 
 
 1 - Answer question
 2 - Select amount of tickets
 3 - Payment page (just billing address needed) and auto save card details. 
 We will also add a ‘how to play’ tab for people to see
 the process, this will be a simple page with text and graphics. 
 We need to make sure that once tickets are selling that its easy to put them in a numerical order on a spreadsheet, maybe via order number and name. This needs to be secure as if there is a problem and we lost the entry names or numbers then it is a major problem. This section of the site will be inspected by UK lawyers to make sure it is above standard so there also needs to be a section for T&C to be read and also selected at checkout.
 
 The above is the main outline of the project other than minor layout changes & graphics which will be added as we work on the development. A recommendation on platform would be great, we have used wordpress in the past and like the ease of use but it needs to be done professionally to get away from the off the shelf look. Other options would be Shopify, CS cart or similar?