Old Gentoo upgrade

Hello,
I was asked to upgrade a Gentoo server. It probably was not updated
for a few years. There is 10.0 profile with portage 2.1.6.7. After
every 'emerge' there is a suggestion to upgrade to 13.0 profile.
I am fairly novice to Gentoo, and this server is constantly being used (e-mail).

First I started with:
  # emerge --sync
  # emerge -1av sys-apps/portage
There was a message:
 emerge: there are no ebuilds to satisfy "dev-lang/python:2.7".
dependency required by "sys-apps/portage-2.2.7" [ebuild]) (dependency
required by "portage" {argument])".

Supposing that my installation is too old to upgrade that way I
decided to follow strictly method described at
   https://wiki.gentoo.org/wiki/Upgrading_Gentoo#Updating_old_systems

Stage3 was downloaded from
http://gentoo.mirror.pw.edu.pl/releases/x86/current-stage3/)
Unfortunately, after the last command from this guide:
 # ROOT=/mnt/host emerge -1v portage
there are many messages such as:

!!! Section 'gentoo' in repos.conf has location attribute set to
nonexistent directory: '/usr/portage'
!!! Section 'x-portage' in repos.conf has location attribute set to
nonexistent directory: '/usr/portage'
!!! Invalid Repository Location (not a dir): '/usr/portage'
....
!!! /etc/portage/make.profile is not a symlink and will probably
prevent most merges.
!!! It should point into a profile within /usr/portage/profiles/

I felt completly lost. I found repos.config only in
/mnt/build/usr/share/portage/config, but there is no 'x-portage'
section.

Is there something wrong with this guide?
May I ask for any hint what to do next, and more generally - how to do
the upgrade in the safest possible way?

English is a foreign language for me, so please forgive my language
mistakes (if any).

Every remark will be highly appreciated.
With many thanks in advance,
Leszek Pankowski