It's a good starting point. But whoever said that was assuming you could gradually build up your arena over time while in D.IV. If you want to promote almost immediately to D.III you need to be pretty close to the finished product, which I've heard is more like 15k seats. Your plan was to be in D.IV for two seasons, there's no way you could add 8k seats and buy yourself a championship-caliber roster. Sorry I wasn't more clear.
Training bigs isn't necessarily a bad idea. However, bigs without secondaries don't sell particularly well on the TL, but bigs with secondaries sell extraordinarily well, especially with good HA/PA/OD. Handling and passing help avoid turnovers, and everyone loves OD. Not that JS/JR/DR aren't important, but I think the other three are what people really get excited about seeing on big men. So training bigs and training secondaries aren't bad ideas. I paid way more to ensure all my players have above-average secondaries.
Here's my advice: do everything in your power to win your league so you can get the attendance bonus when you promote, then next season in D.IV focus on getting Delong and Coleman 48+ at PG each week. Train passing and mix it up with OD once in a while. On short weeks, train 1v1 forwards. On weeks with must-win games, go back to training primaries. You should be making a lot of money especially if you have the attendance bonus, so keep constantly expanding yourarena as fast as possible. You'll have that attendance bonus most of the season, so you'll want to build new seats as fast as you can to take the most advantage of it. Since I know you're on the offsite, use the attendance tool to figure out the best prices. Put at least $10K per week and at most $20K per week toward scouting the draft.
Assuming you aren't in a terribly hard league, you should avoid relegation with this plan, but if you relegate, fine. You'll have made some nice money/expanded your arena, you'll be a more experienced manager, and you'll have made your trainees infinitely more valuable for the inevitable day when you have to sell them, and you'll have put yourself in a nice position for the future. Plus, you should have a high draft pick and a chance to land a stud draftee. This is what I did. It wasn't all that fun for the season while I was doing it, but now I'm way better off for doing so. I'm 11-2 in my first season of my second run in D.IV, a title contender for the next several seasons (none of my key players are older than 28, so even though the Cresleys will probably win this year, I have a good chance next season), and I was able to draft a stud prospect in Paul Ogle, who has MVP potential and started with 59 skill points. Who wouldn't go for that?
Hope this post gives you a little more reasons for optimism than my last post, which was kind of depressing.