Write some Software

Working with a new client of mine I want to use modern principles, methodologies and software development technologies (such as OO, and cloud-based systems?) to build flexibly and robustly what in the past I did for other clients with clunky error prone botched-together Access and Excel 'programs'. I want neat, robust, tested code and I'd like to learn enough as we go along to follow the process and to understand what does what (though not necessarily enough to be able to write the code myself). I'd like if possible to avoid having to start from scratch by having my freelancer first help me explore what might already be out there (maybe in open-source, possibly for unrelated implementations) that could be adapted and built upon. The first element I want up and running fairly quickly involves taking an electrical tender Schedule of Works (usually arrives in an Excel spreadsheet in various configurations), to import the project elements that each have requirement lists, to apply our own categorisation codes such that we can identify and re-order the work and materials required for the project in a way that works for us, to enable us to put procurement costs and apply mark-ups and then to be able to export with the relevant information in back in the original order (and if possible in the same format) in which it arrived. Modules I'll want to add to this - hopefully with the same freelancer if the first part works out well - include: Generating working Schedule of Works for contracts won allowing for element removals and agreed extras to be added as jobs progress. 'Valuation Sheets' (from the SoW) for inputting and generating for client's approval the percentages of job elements completed at invoicing intervals and from that, once approved, invoicing in accordance with approved valuation. Exporting invoicing data for import into accountancy software package. Inputting cost data for project costing, exporting purchase invoice lines to be imported into accountancy software. Time sheets and pay runs, incorporating UK Construction Industry Scheme tax deduction management - which is fairly straighforward - and exporting to import into accountancy software. And more - which will become apparent as I get to know this electrical contractor client's business and as he gets to use and know my process designs and our work (my client, not my client's clients referred to above)! It may be that we'll end up with something mysql-based but I'm liking the idea, providing the development overhead isn't too high, of developing in a way that's more conducive to changes in requirements than a relational-database-based system. I'm inclined to do everything in the cloud in AWS because of security, to enable eventual remote, cross-platform access to various modules and particular data according to permissions etc. I'd be happy to pay forward more than half my hourly rate when working together with my freelancer to develop these. If / as and when we can deliver modules that improve things for my client, he has already said he'd be more than happy to pay a higher hourly rate and a freelancer that is helping me deliver will accordingly deserve more.