Flat nodes don't impact upgradability but their benefit is greatest for full planet imports.
Since you do not need a continuously running tile server - you can afford downtimes while updating - the most space-efficient way to do what you want is
1. if your area of interest is rectangular, goto 2; else determine the area you want and create a "poly" file for it (e.g. draw polygon in josm, save as .osm, use osm2poly.pl - or use another method)
2. download europe-latest.osm.pbf from Geofabrik
3. use osmosis (or osmcut or osm-history-splitter) to cut out the area of interest from the Europe file
4. import the resulting file with osm2pgsql using the `--slim` and `--drop` options which leads to a non-updatable database and makes the database unusable during import
5. whenever you want to update, repeat steps 1-4.
Alternatively, if you *do* want uninterrupted service, follow steps 1-3 above and then
4. import the resulting file with osm2pgsql using `--slim`
5. when you want to update, repeat steps 1-3 above and then
6. use osmosis with the `--derive-change` option to compute the difference between the new file of interest and the last one and save to an .osc file
7. use osm2pgsql with `--append` to load that .osc file
There are other options, like consuming the Europe diffs from the Geofabrik site, but that would lead to unnecessary data in your database that you would have to remove from time to time.
Assuming for a moment that you need to
The recommended procedure for your particulare
