This is a high risk strategy if you only have a 10% deposit. There are lenders at many auctions that will arrange "interim lending" (otherwise known as bridging loans) which are very expensive but would help you to secure the finances while you worked out a more conventional mortgage. However, if the high street lenders decided that the property wasn't mortgageable or that your plans didn't stack up financially, you'd be stuffed.
Conveyancing has nothing to do with access to the property, it is the process by which solicitors check that the property is legally in order and the title can be transferred to you. This usually takes place (over many weeks) before exchange of contracts and the solicitor will typically make many enquiries of the seller's solicitor regarding, for example, shared access/boundaries/permissions for any extensions etc etc and will want all these questions answered before allowing you to exchange. As you know, exchange at an auction happens when the hammer falls so, if there are any legal problems with the property that you haven't picked up beforehand (either by reading the legal pack yourself or paying a solicitor to do it for you), then you are stuck with them. Many auction properties do have legal problems.
If by conveyancing you mean a survey, you can instruct a surveyor who will get the keys from the auctioneer and carry out a survey for you. Bear in mind that you could spend several hundred pounds on this only for someone else to buy the property on the day. If you wish to knock it down to build an new house, is it worth having it surveyed though? And if you are only able to raise 10% of the auction price, how will you pay to build a new house?
Auctions are basically there to serve cash buyers or people who work in property. I think you should proceed with caution on this one.