>>1780337you won't get a cheaper bike by buying and assembling it yourself. You may get a gravel on a budget by modifying a old hybrid tho, but it depends on your local used market.
Ideally you want a 2/3x8/9 hybrid with a rigid fork that can clear 700x42 or whatever tyres you'd like. Those bikes are usually pretty upright and take well a dropbar conversion.
then you want some brifters, which are pretty expensive, getting something decent used would be great. Cheap new you have microshift advent and adventx or I guess some sensah, assuming you want a clutched derraileur for a 1x build.
next you need to think of brakes, if the original bike has v brakes, you need to get mini vbrakes or a pull ratio adapter, cantilevers would take some extra stuff. If it had disc brakes, you need to get some compatible calipers, like avid bb7 road or whatever you can get.
to get 1x if the original crankset has bolts is trivial, you just remove all chainrings and replace by an appropiate narrow wide one, if not, you need a new crank that can take one.
that would be the big picture stuff for a hybrid conversion. gonna do a best-case example to illustrate:
you get a merida speeder hybrid with disc brakes(I am making this up), 3x8 with bolt on chainrings and rigid fork. you also get some cheap sora 2x9 brifters. you need to buy some bars, road compatible calipers, a 9 speed casette and chain, if you want to go 1x, now you need a sunrace rdm900 derraileur and a narrow wide chainring (40 t?), and the casette you get may be 11-42 or something like that. you can also leave it at 2x9 with the same derraileur and adjusting the front derraileur to the two biggest chainrings. total cost: bike + brifters + chain + casette + chainring + calipers + derraileur + handlebar= pretty expensive unless you get great deals on stuff.
