Tips to select the pattern size to use 
https://youtu.be/1IrV1w_ihwE?si=0WhwRyqdUJ0aU3OA
How to mark balance lines for fitting on  your pattern.
Part1
https://m.youtube.com/watch?v=-VpZ6YIrycg&t=0s
Part2 which shows how to work with a flared pattern.
https://youtu.be/urfLcvNLIDo?si=ey4333XGtk4pMqc2


How to check the pattern measurements so you can  compare with your actual body  measurements. 
https://youtu.be/U7bU04fuC-M?si=fqBWt97ouUWQc9Ig
And
Prepare
https://youtu.be/dv8bNng19-A?si=3zXmb8ky-fDXuU5i

Initial fitting- How to asses issues https://youtu.be/EMsnw1S2g14?si=oATaXsA8IwxAKYE6

Hope some of the information may be useful to you . 

It's not a model I've used, however from a quick look at the manual here https://www.manualslib.com/manual/1342150/Frister-And-Rossmann-Euro-21.html?page=17#manual
It looks like a straight forward machine for a novice.
Drop in bobbin, adjustable stitch length & width, snap on feet
If newly serviced it's probably well worth considering. 
Perhaps take a look at the list in the manual for the original accessories and check with the seller that the standard foot and buttonhole foot are both with the machine.  They're the  ones that matter most.

. . . My budget has been increasing with every new suggestion 
I'm now looking at the Babylock and the Juki. They are both at the show so I can have a go with both lots.  Looking forward to it. (excited squee).

I am sure you will be delighted with either a Juki or a Babylock. 
Will you be getting a model with the bigger motor rather than the smaller models?
It's really worthwhile getting one with the bigger, more powerful motor, especially for working with fabrics of different thickness and layers.
The motor has a tough job to provide needle punching power and also drive the cutting knife and the feed dogs at a high speed. 

I've threaded and worked with  all sorts overlockers - don't mind manual threading and do prefer to be able to manually adjust the tensions. 

Currently I have a 14+ year old Babylock Evolution (air threading)  and my 30+ year old Bernina (manual threading +knob dials for tension).

If I get another overlocker it will be a Juki overlocker only and a separate coverstitch - probably a juki industrial  which is a a dream to use and costs less than the Euphoria irxa couple of other higher end brands of coverstitch. 
 
Currently I think Juki offer excellent value for money in terms of performance whichnis comparable with the higher end of Janome, Bernina or Brother. 

If you fancy a trip down a rabbit hole about small run cloth weaving, just watch some of Sam's videos on Instagram by   Woveninthebone

It's a great insight into the process involved in setting up the loom and the effort.  So much goes into making the fabric . . . and then we cut it up to sew pieces together. 

https://www.instagram.com/reel/C4G6LBLolim/?igsh=cHRyY3V1cmxsYjk5
 For more videos-
https://www.instagram.com/woveninthebone?igsh=cms2Y3gyMjZ3eGZu

. . .  only to discover the button guard has come away from the front.  I have ordered some new buttons because there’s no way I can get the current ones off and will have to bodge a new button guard 
 [ You are not allowed to view attachments ]  

What an infuriating thing to happen after all that work and your perfect top stitching.
I probably would have sworn some very, very nasty words.
Will you replace the button extension or use it as a design opportunity for a creative solution?

@RachelB
As you are fitting solo, I wondered if it would help to do as much as you can on the pattern before the first fitting toile/muslin.
Alexandra Morgan has a useful collection of fitting and pattern making videos on her You Tube Channel which is where I get the links in ths post. 

https://youtu.be/dv8bNng19-A?feature=shared

https://youtu.be/EMsnw1S2g14?feature=shared

I believe that clearly marking the balance (reference ) lines on the toile is essential to be able to diagnose fitting issues and establish a a good fit. 
https://youtu.be/-VpZ6YIrycg?feature=shared
https://youtu.be/urfLcvNLIDo?feature=shared

Here on a pants pattern  she shows how to prepare the balance reference lines and explains some of the adjustments to prepare the  pattern before making a toile/muslin.
https://youtu.be/qZ0n_73H4h8?feature=shared

https://youtu.be/uQnPToWwb8c?feature=shared

For fitting a bodice, the shoulder is one is the first fitting issues to assess and deal with.  It's always a good idea at some stage to work out what your own shoulder slope is.
Also,it's helpful to know how to work out what the shoulder slope of a pattern is and then how to alter the pattern to revise the slope.
This video is about working out the actual shoulder slope of a sewing pattern.
https://youtu.be/yLId3ivN92w?feature=shared
This video is about how to find the degree of slope of the shoulder slope line on the pattern.  Toward the end of the video, she touches on working with a front pattern angle and a back pattern angle which can be useful when working with a pattern that has been adjusted for a forward shoulder or slanted for style shoulder lines. 
https://youtu.be/yLId3ivN92w?feature=shared

Once you know what your shoulder slope is, including any forward shoulder adjustment, it's worth making a template to save yourself time and trouble with any future patterns. 

And in case a forward shoulder adjustment is of interest.  She shows 2 methods in the linked  The first is the common visual line shift and the second is a combination adjustment that includes an armhole shape fitting adjustment at the across back and across chest levels.
https://youtu.be/jSwONVfA0Ow?feature=shared