I've found that an adequately fitted ship is more important than a more expensive one (given the same roles), so I'd say get a FDL. Even if that weren't the case I'd still recommend that you go with the FDL because, as you said, you don't have the funds to outfit a python for anything fun. Fun is what we're playing Elite for, so if you don't enjoy trading don't make the next few weeks a chore by forcing yourself into a trade exclusive python. So yeah. Either go with the FDL, or keep saving in something you enjoy until you can afford to get an acceptable loadout on a python.
Personally, I like the python a metric fuckton and will probably skip the FDL until after I'm comfortably cruising around in a conda/corvette, but then gunboats put a huge smile on my face. If fighters like the vulture do it for you, but you're after an all-around upgrade, then get yourself a FDL.