hey @mage sorry I saw this and meant to post earlier but havent had a chance before now. Re base, make it as light as you can for the weight of what’s going on top as the weight of the piece will add up once you start adding to it. I don’t know if you have a dedicated craft store near you but if you do then those places can be gold. MDF/foam core (Sold for photo frames)  is great for walls – just cut to size with a sharp hobby knife and rules, glue with PVA and use pins t hold in place until it sets or use a hot glue gun if you want an instant bond. Balsa wood or coffee stirrers for fixtures like door and window frames. Craft stores may also have supplies for making dolls houses (usually in 1/12 scale?) so while the furniture might not work you might be able to get printed paper or stickers to use for tiles or wooden panelling for the floors of the house. If you are super-keen someone suggested once small doll’s house woven rugs can work for interiors in gaming too. Given the 1920’s setting you might be able to find some printed paper for scrapbooking/origami which can be repurposed as wallpaper.

With regard to furniture, I’d say don’t worry too much – you can most likely come up with scratch built representative equivalents form paper/cardboard/coffee stirrers/modelling clay. s long as it represents what it’s supposed to be then you should be able to get away with it. If you are keen for something more fit for purpose and have the time/inclination then zealot minis do a bunch heartbreakingly beautiful dungeon furniture which you might be able to use in a more modern setting. Or there is the mantic stuff that has just been released too – tavern/library/wizards sets might be appropriate and would provide scatter for other terrain projects later.

Failing that if you have access to a reasonable quality colour printer maybe check out paper terrain options (Fat Dragon games or similar? if I recall there were a bunch of printable terrain options on Drive thru RPG?) – would save you needing to paint/texture the facade of your building and there may even be furniture options.

The craft sections in discount shops/pound stores are also good for little bits and pieces. necklace charms can occasionally come in surprisingly useful shapes (Ships wheels and anchors etc) so you might spot something useful. Best of luck 🙂