House construction planned, what sums must one reckon with today?

Oops. I just quickly entered Rabenau into Google and this from Saxony comes up prominently ;)

Since I am currently self-employed, the income is highly variable.

That could be a problem for the bank! Self-employment usually needs to be longer and permanent. Especially fluctuating numbers are usually non-negotiable. Then comes the category "online shop" as a one-hit wonder... I would first speak to the bank to see if you have a chance.

Doesn't look so bad at first glance... 4.1 Type of structural use in the planning area

None of this is in the development plan, but under Par 4 land use ordinance. Usually, the restrictions are then stated in the development plan. Because exactly these daily deliveries and regular trucks are the nuisance to the other residents. As for the price: it depends on what you understand by upscale. Since normal standards cost around 3-3.5 thousand per sqm, calculate with 4-4.5 thousand.

is stated in the development plan, the link has since been removed. Here is the corresponding excerpt (I had not read everything before) copypaste is unfortunately not possible because the PDF is protected (and I currently don't feel like disabling the protection) It is the development plan of the municipality of Rabenau, district Geilshausen. Residential area "Im Löndörfer Feld"

that means actually allowed but here specifically not (and definitely not)

That could be a problem for the bank! Self-employment usually has to exist for a longer and permanent period.
Especially fluctuating figures are usually non-negotiable. Then there is the branch "Online-Shop" as a one-hit wonder… I would first talk to the bank to see if you have a chance.

The land would be purchased without taking out a loan.

I see it the same way, first talk to the bank about how much credit can be obtained at most.
But since there seems to be a lot of equity, it should turn out rather positive.

Permitted are: 1. Residential buildings, 2. shops, taverns and restaurants serving the supply of the area, as well as non-disruptive crafts, 3. facilities for church, cultural, social and health purposes. Exceptionally permitted uses are: 1. Accommodation businesses, 2. other non-disruptive commercial enterprises, 3. administrative facilities, 4. horticultural businesses, 5. gas stations

"other non-disruptive commercial enterprises": To me, that feels very vague. But maybe it is a standard wording intended to keep loud industry away. Trucks are definitely considered disruptive by many people. Families with children will certainly want to live there.

What kind of trucks are we talking about here? More like 7.5 tons or 40 tons?

Actually, there is only one thing left: make an appointment with the city, present the project, and ask if that is okay.

Skimming also indicates a mixed-use area?! According to this, a commercial use could of course be feasible. However, I also read about the slope and problems with the establishment. It is probably also the reason that building has become expensive, that those wanting to build have backed out again, but certainly also the reason that it is not an easy area (slope, groundwater).

"Other non-disturbing commercial enterprises" are explicitly prohibited! (See text excerpt). The formulations in development plans are often deliberately or out of incompetence expressed in a difficult-to-understand manner in my opinion :-(