Your answer are in the right direction. However, let try to refine it further to helps you arrive at an answer that wows :-)
There are two types of business models in online grocery business- Inventory based and Hyper local based and that's the set of lliabilities and infra requirements vary.
1. Intentory based : This is an expensive, integrated e-grocery model as it involves managing own warehouse with cold storage and a fleet of refrigerated trucks thus offering quality fresh groceries.
Liabilities : Huges Investment to Serve and Scale, Very High Working Capital, Risk of Wastage because grocerries are preshable items, and Higher operation cost accountable in terms of procurement, quality check, storage, inventory planning.
2. Hyper local based : Here companies do not own infrastructure, rather they are simple aggragators manging order and have tied-up with local stores for order fulfillment. With zero inventory and storage costs, the online aggregators just focus on customer acquition and branding/marketing.
Liabilities : Here the main liabilities are operaitonal eg. Quality and Timeliness for goods delivered
Hope these help you determine the liabilities associated with different business models.
Coming to your second question whether to buy or use an existing business model completely depends on the business model and future roadmap. Here are few points that might help decide which on the risks associated with each :
1. Existing Infra : Esxisting Infra would fail if it can not be scaled as per the volumetric/transactional requirements. Exisitng infra is not competent to new services and partner integration
1. New Infra : You would need a new infra if there is an expected change or evolution in business model and services both from regulatory and operational point of view eg. loyalty management driven cross industry partners, new services like grocerry wallet (pay and earn throgh wallet).
Hope these help. Please feel free to connect if you want ideas and insights into new business models.