You have a good foundation of experience with your current bike. That's a great start toward finding something that you'll really like.
Though we don't recommend products there are things to think about when looking for a bike.
The link Swifty posted in comments is an article with many good thoughts.
Summarizing the thoughts:
- Think about what kind of riding you will be doing, not just on the charity ride but beyond that. There is a range of bikes that are designed for functionality between extreme off road and extreme on road with every flavor in between. The key will be to settle on a type of bike that best fits your need.
- Comfort - the bike should fit you and feel comfortable. If a bike isn't comfortable you are less likely to ride it. It's possible for a really bad fit can injure you.
- Generally, bike manufacturers carry a line of bikes with a model in each type and price point. Once you have identified the type of bike that fits your need for a particular price comparison shopping becomes easier. It's a matter of comparing similar types of bikes from different manufacturers for a particular price.
- Visit your local bike shops. You should find people who like to talk about bikes and won't pressure you into buying something. They should encourage you to test ride different kinds of bikes so you can gain enough experience to know what feels comfortable to you. If the first bike shop you visit isn't like this try another one.
If you select a new bike based on your type of riding, fit, and feel I'm sure you can find something that will be fun to ride in your price range.