- your question is not very clear
- page layout of your document is unknown (how wide can table be?)
- in MWE (Minimal Working Example) you provide a dummy data, which apparently not mimic real data (regarding their lenght), so it is hard to help you in table design
- regarding problem with multi column cells, @Patrick Ferguson solve your problem in his answer (+1)
- it is not clear, why you have different column headers in the first part of your table and at other parts
- for numbers in last three columns is sensible to use
S column type defined in the siunitx package: wit it numbers are aligned at decimal coma
One solution for your table, based on retyping showed desired table lookout in the first image in your question), can be:
\documentclass{article}
\usepackage[margin=25mm]{geometry}
\usepackage{booktabs, longtable, makecell}
\renewcommand\theadfont{\small\bfseries}
\renewcommand\theadgape{}
\usepackage{siunitx}
\usepackage{lipsum} % for dummy text filler
\begin{document}
\lipsum[1-4]
\begingroup
\small
\sisetup{table-format=6.2,
output-decimal-marker={,}
}
\setlength\tabcolsep{4pt}
\begin{longtable}{@{} llll SSS @{}}
\caption{Eine Tabelle in Überlänge.}
\label{tab:long} \\
\toprule
& & & & \multicolumn{3}{c}{\thead{Umsätze in EUR}} \\ % this the key addition
\cmidrule{5-7} % the underline of Umsätze...
\thead{Firma}
& \thead{Ort}
& \thead{PLZ}
& \thead{Straße}
& {\thead{2013}}
& {\thead{2014}}
& {\thead{2015}} \\
\midrule
\endfirsthead
\caption[]{ -- continued from previous page} \\
\toprule
& & & & \multicolumn{3}{c}{\thead{Umsätze in EUR}} \\ % this the key addition
\cmidrule{5-7} % the underline of Umsätze...
\thead{Firma}
& \thead{Ort}
& \thead{PLZ}
& \thead{Straße}
& {\thead{2013}}
& {\thead{2014}}
& {\thead{2015}} \\
\midrule
\endhead
\midrule
\multicolumn{7}{r}{\footnotesize\itshape{Continued on next page}} \\
\endfoot
\bottomrule
\endlastfoot
%%%% Table body
Donau Buroeinrichtunegen
& Linz & 4020 & Schubertplatz 9
& 51560,00 & 56716,00 & 62387,00 \\
Burocenter Tirol
& Innsbruck & 6020 & Innstrase 234
& 480536,00 & 528589,00 & 581448,56 \\
Carynthia Firmenauststatittung
& Klagenfurt & 9219 & Kaiser-Frantz.Str. 1806
& 199120,00 & 219032,00 & 240935,20 \\
\addlinespace
Donau Buroeinrichtunegen
& Linz & 4020 & Schubertplatz 9
& 51560,00 & 56716,00 & 62387,00 \\
Burocenter Tirol
& Innsbruck & 6020 & Innstrase 234
& 480536,00 & 528589,00 & 581448,56 \\
Carynthia Firmenauststatittung
& Klagenfurt & 9219 & Kaiser-Frantz.Str. 1806
& 199120,00 & 219032,00 & 240935,20 \\
\addlinespace
Donau Buroeinrichtunegen
& Linz & 4020 & Schubertplatz 9
& 51560,00 & 56716,00 & 62387,00 \\
Burocenter Tirol
& Innsbruck & 6020 & Innstrase 234
& 480536,00 & 528589,00 & 581448,56 \\
Carynthia Firmenauststatittung
& Klagenfurt & 9219 & Kaiser-Frantz.Str. 1806
& 199120,00 & 219032,00 & 240935,20 \\
\addlinespace
Donau Buroeinrichtunegen
& Linz & 4020 & Schubertplatz 9
& 51560,00 & 56716,00 & 62387,00 \\
Burocenter Tirol
& Innsbruck & 6020 & Innstrase 234
& 480536,00 & 528589,00 & 581448,56 \\
Carynthia Firmenauststatittung
& Klagenfurt & 9219 & Kaiser-Frantz.Str. 1806
& 199120,00 & 219032,00 & 240935,20 \\
\addlinespace
Donau Buroeinrichtunegen
& Linz & 4020 & Schubertplatz 9
& 51560,00 & 56716,00 & 62387,00 \\
Burocenter Tirol
& Innsbruck & 6020 & Innstrase 234
& 480536,00 & 528589,00 & 581448,56 \\
Carynthia Firmenauststatittung
& Klagenfurt & 9219 & Kaiser-Frantz.Str. 1806
& 199120,00 & 219032,00 & 240935,20 \\
\end{longtable}
\endgroup
\lipsum{6}
\end{document}
If your real table is wider than table above, than is sensible to consider to write some cells contents (for example in the first column and maybe in the fourth too) in two lines. IN this can be of help xltabular package, which is combination of the longtable and tabularx package.
I'm trying to make a table like the one above
