Not to worry, life brings these bad news from time to time :-)
Go ahead and glue it all up. I'm hoping to collecting it in person at some point. I'll give you enough warning when I can visit you and talk bikes :-)
I'm not sure about solid timber or MDF. If I can get 5 years out of it with solid timber then that would be perfect otherwise MDF.
The pound shop, you never know all the things one can find there
If you are increasing the distance or the pace you have to allow enough time to rest. I went through something similar years ago. Once the old body gets used to then you are fine.
I'm looking forward to see your new bike