Welcome to the forum!
Thanks so much for your letter. A surprise party is a great idea and I'm sure (that)(this is optional - I'd omit it.) Anna will love it. You are(more informal to use the contraction:) You're right - your house doesn't have enough space and (it is) it's a good choice to book the Majestic Hotel and give the party there. But you might find it expensive, and if you do, just get in touch with some friends and see about another place to have the party.
(It is) It's quite difficult to think
inof a present for her, but she told me once that she wanted to go to Brazil, so we (can**) could think about itthat.
I know that you need help with the preparations the day before the party, but unfortunately I will be busy - I have so much to do at work.
I would suggest you
tocall your brother. (he is) He's a good DJ, and you know how much Anna likes electronic music.
I hope everything
** if it was just you who was going to think more about the present, and Brazil, then 'can'. But you write 'so we (can/could) think about that'. Using 'could' does not preempt any ideas that Peter might come up with about how to decide about the present. In short, rather than telling him flatly 'we can', you are making it more a polite suggestion.
VERY GOOD ENGLISH. The red is just polishing it a little.