I think you should be able to find a vintage Bernina 801 or for around £100-140, depending on condition. I would not pay more than £50 for any fix up machine. The only problem with specific vintage models is it can take time before one turns up. Newer Berninas are more expensive, even two-four year old machines tend to cost quite a bit more. Any Nova is around 40 years old. I think my maximum for a vintage model is £300, regardless of brand and accessories, generally £150.
For comparison; a beige (or black) aluminium Singer 201 (201K23) weighs around 10 kg. It's the smoothest work horse around, even today. It's straight stitch only though. Several of the new (current) Singer Fashion Mate models weight around 7 kg, they have all kinds of stitches, but aren't as capable when it comes to heavier fabric and thread weights. Even sheer fabrics like chiffon and slik can act up on these. Ideally we should test sew before we buy.
Many of the simpler machines use feed dog cover plates, and doesn't drop the feed dogs. If you are used to Bernina top models you probably expect a lot of a machine; know that some of the light weights and affordable models some are very happy with aren't exactly what I call dependable or nice to work on. Some of them are well within acceptable, it all depens upon what we expect from the machine.