The advantage of only training trainees is, if there potential is high enough you will get quailty players, if you draft right, for an even price based on scounting through the whole season, beware I joined this game after the season finished & before the draft & the draft was terrible & I'm in the same Division as you for England.
Look on the transfer market daily & sometimes you find quailty players for a cheap price. I found it's best to look on game days as most people are playing there games & watching them & not looking how there bids are doing.
Hope this helps.