NOT all trains need to be REServed-often its only optional. THis varies a lot per country.

Yes, the 1 and only trenlerinde out of TR is that overnite from Halkali to Sofia-as any train does it makes more stops on the way. ONLY in summer this tren will also have more cars that are shunted+more and go to Bucuresti-NOT in febr. You should be able to book a couchette/kushet in that tren at any staffed TCDD counter. The trip to IST-assuming you even want to use TCDD for that, is also included, ON THAT same day only! THis effectively limits it to the TR hi-speed lines from ANK or KONya

Best site for general planning;seat61.com. DO note, as it seems you hardly have any uptodate info, that the classic route via old Yugoslavija is NOT possible by train-this means a fairly long detour via ROMania-HUngaria. Also inform yourself about any visa-rules for EUR and the Schengen area.

There have been several same-same posts here recently about this route, so a search and reading will also tender a lot of info.

Well, probably most convenient one will be to use direct nighttrain Istanbul-Sofia as otherwise you have to get up very early in Dimitrovgrad (if it is on time) or have to worry if the connecting train waits or not (if it is delayed). From Sofia no reservation required to get to Vidin (either sleep there or in Sofia). Then take the train to Craiova and from there with one change you can get to Budapest, from where you have connections to Austria, Germany, Slovakia, Czech Republic, ...