Dear Simon, thank you for writing. It was nice to hear from you and I am glad you are enjoying your stamp collecting.
You asked what does “mixed condition” mean, well generally it means there are some good stamps and some not so good in the group of stamps that are being offered for sale.
As a guide I would suggest you should be able to see all the stamps you are thinking of buying especially if you do not know the seller. and….. just because the catalogue value is high doesn’t mean that to get it at a low price is a good bargain.
Perhaps they might be useful to collect for comparison of postage cancels or colour shades, or perforations but the ones you showed me some images of look like they may have rust on the stamps or at least on the page they are kept on.
That is not good as the rust can spread in your album so I would be very careful with stamps that look like that with a yellow tinge to the edges especially if they are not described with rust on.
You mum has invited me to dinner soon so i will bring some stamps and show you how to wash them and look after them. Keep enjoying your stamps it is a wonderful hobby see you soon Love Uncle Michael