You've gotten some good advice already, but here are my 2 cents.
1. Don't think that doing well in DV means anything. DV can be won with scrubs if you just manage enthusiasm. I suspect that at least half of DIV teams don't manage enthusiasm so it should be easy to stay in this league. Early in the season, you can always lose a couple of games if you play TIE since your enthusiasm is reset to 5. Also, I think it can be easy to win DIV if you get one stellar defensive guard, and 2 old inside men (at least 12 IS, and 12 ID) and play LI. DIII seems to be where the significant competition begins (at least in the US).
2. Training seems overrated to me (as compared to just using the TL). Practically speaking, it is impossible to 2 position train in DIII and above unless your trainees are quite advanced already. And, even 1 position training will be difficult to win games unless your trainees are again quite advanced already. So, it seems to me that you have to sell and rebuild your team at every level. I now believe that you'd be better off buying pretrained 20-23 year olds and just finish their training if you want to be competitive at higher levels. Perhaps with cross training, it would be even better to just train FT and not bother with trainees and keep all that extra trainer salary to buy players?
3. Arena expansion is necessary long term, but the ROI per unit is very small since you can just raise prices on your existing seats, and added seats reduces the price that fans will pay for those units.
Anyway, stay in league DIV and try to get into DIII. DV is boring as heck and most DIV leagues don't even have any activity on their forums.