What I did, which is not necessarily the best method I would assume, is focused on building up my arena bit by bit like Azariah suggested. My path to success in Div III took a couple attempts because I just wasn't ready long-term financially. My first venture up from Div IV lasted four seasons (I actually made the semi-finals in my second season - but that was due to a weaker conference). I relegated back down and came back the next season and found my groove. Because of salary increases due to training and not having the economic base to sustain it, I did have to weather two below par seasons before I was able to build up into a Div II team.
The merch store is great - but I think it's better if you get a solid arena receipt built up first so you can really capitalize on it. Having said that - your team is currently (salary wise) much better off than my first III team was. Three of my starters were 12k or less (two trainees swapping out as rookies that year - one of them now my boy Kurt Brown). You're pretty far above the floor. If I were you, I'd let the first part of the season play out to see where you seem to be heading and make a decision to cut salaries and play the relegation roulette or push for playoffs and a first round exit. But I'm also known to take the scenic route to get to where I should be headed so...take it as you will.