All longboards will feel harder to turn after a shortboard - its not a totally different technique (weight back etc) but a shortboard can let you get away with just leaning or grinding a turn, a longboard wont. So dont expect to find a longboard that is as easy to turn as your current board
In any case, a 'modern' longboard (I assume you mean a high performance one) arent aimed at a 'calm way of surfing' - they are more shortboard style in a longer board, light weight, more rocker, maybe harder or thinner rails. Whereas a traditional log is designed to be relaxed, able to cruise past sections and so forth. Heavier, flatter rocker, soft rails, thicker etc.
Most people ('most'!) prefer the traditional style, although nowadays there are a lot of boards that are sort of in between eg slightly narrower or slightly lighter (eg epoxy construction) . But up to you what kind of style you want; it might be that moving from a 6'4 to a longboard for the first time makes a longboard seem very strange; after a while you might find the issues will go away
I dont know what
you can get in Spain.
The Torq boards are generally a good lower priced choice eg
http://www.torq-surfboards.com/long-90.html or something that is along those lines (eg 9ft x 22 or thereabouts, low rocker). NSP make some good ones as well; but obviously there are heaps of other manufacturers or shapers - everyone does a longboard and most of them are fine.
Probably just try to avoid a specialised nose rider at this stage; most other 9ft or so boards will be fine