Well that true. Many of the promoting teams are buying quite a lot of expensive players etc. But hey, you don't need to feel left out, because I'm not buying the expensive players either.
Your current team is strong enough to make it to the playoff. The first thing to do really, should be to expand the arena, which is exactly what you're doing. If you know that you're not going to relegate, you could simply wait until the arena is completed (the size you want it to be), then earn a lot of income, then start buying expensive players for a real competition. In my opinion, the person who gets to complete the arena first wins than the person who has expensive players but small arena, because in the long run, the person with a complete arena will advance quicker.