Hi and welcome,
Your cunning plan looks, to me, like it could be costly. You plan to buy something and change all the bits - perhaps buying something closer to what you want initially will reduce the costs. I have a doubt that your budget would be enough to cover your plans.
You could buy a frame and forks for around £100
Groupset (i saw a 105 with wheels tonight) £120
Additional parts - £80
That would keep you in budget - but perhaps with a lower standard of parts than you want.
My opinion, and mine only, is that you should try and source a bike which suits your needs when you buy it - without the bother of all the changes. You can upgrade parts etc. if you wish, in your own time and it should still work out less expensive. You mention trying the bike - you will be trying a bike with drop bars when you plan to change them - this seems peculiar. You should ensure that the frame fits before you buy - which is obvious. I do not know if the fitting for straight bars is the same as for drops but i would presume it is the same.
Good luck with your build, and i would be interested to see what you come up with - post in readers bikes when you start.
Welcome to the joy of RB
Thanks a lot for your reply !
500 euros was what I had in mind for the frame + groupset only! My "all inclusive" budget is around 1000 euros, or maybe slightly more since I can spread the parts' cost over a few months...
I didn't think of the drop bars fitting issue though... But since most "fitness" are based on a road frame + straights I'd assume that It's the same.