Lemon for me. I decided on a lemon for my patio because I thought that even if the fruits turned out unattractive they would still have every day culinary use and they did. Quince and lemon marmalade to die for. If you are restricted for space or climate go lemon.

You could try doing a 'google search' for information on what is causing the damage, how to eliminate and protect your tree, or the RHS - Royal Horticultural Society may have information on their website which could be helpful.

A better photo would be helpful, all I can see is 1 yellow leaf amongst a bunch of healthy looking ones, leaves do get shed by any deciduous tree - this is what scab looks like: Personally I'd keep it, you can spray it which would help but you're probably not going to eat the fruit anyway unless you're intent on making crabapple jelly? I have an old Bramley apple tree in my garden, it's completely rubbish, covered in canker but it still has blossom, the apples are still edible & it allows me to grow climbing plants up it so it serves a purpose for me. Taking a tree out of a garden really needs thought as it will totally change the space & as you say, shade is really welcome (unless it's a Leylandii tree, in which case it should be torn out & burnt, possibly with celebratory dancing round the fire...)

I think you could create a mini orchard on your front lawn, but how big your trees will get depends on what rootstock they were grown on. Do they still have their labels on? Basically, standard fruit trees grow to 25 ft tall and as wide, semi dwarf grow to 15 ft and as wide, then dwarf fruit trees usually grow to a max of 10 x 10 feet. I only know this because I recently bought a dwarf Apple tree, as my garden is far too small for a standard fruit tree. It looks like a spacious area, so you could add more trees, but careful what you choose as some are huge. A nice smaller tree is the Mountain Ash (Sorbus) which has lovely leaves and gorgeous berries in the autumn which the birds love. Laburnum is a small tree with beautiful hanging yellow flowers in late spring. Acers (Japanese Maples) are smaller trees in stunning shades, but they prefer dappled Shade. My favourite tree of all time is the beautiful Himalayan Birch (Betula) which has stunning white bark on the trunk and branches. The multi-stemmed one is particularly attractive. It is a biggie though - 50 ft tall x 25 ft wide. This may not be what you’re thinking, but once established they don’t take much upkeep. Heres some pics