Hello SeppWeller,
what is your current status - we are also at a similar point right now.
Ground reserved - foundation slab and earthworks as an estimated price - house is open - budget 450K
I can tell you it is extremely tight!
Regards Sepp
Since SeppWeller has not been active since the end of May, I do not believe he will read the question.
My general tips if you are working with a rather tight budget:
1. Clarify ground risks early! That means: soil survey (foundation recommendation, disposal class). Drainage connections (especially depth). Connection fees in the area. With this information, you can rule out some of the most unpleasant surprises in advance.
2. Set a priority list: must have, nice to have. What should be done immediately, what can possibly wait a few years?
3. Some work can be done relatively well in self-performance, or savings can be made through external contracting (e.g. laying vinyl or laminate, wallpapering). Others should rather not be outsourced, even if it sounds tempting (especially whenever warranty is a big issue; example foundation slab).
4. Even if it hurts, still keep a buffer. Something always comes up.